Is Your Chicago Training on Track? The 3-Signal Check¶
Your easy runs have been 35–45 seconds per mile slower than your goal marathon pace. Your heart rate has been running 8 bpm above the zone you wanted. You missed one long run, and three of the last five weeks logged slightly under the mileage target. You have been telling yourself it is just the heat.
Maybe. But "it's just the heat" is the wrong conclusion when you're drawing it from intuition instead of data. Four to six weeks into a structured training block, you now have enough accumulated history to run a real diagnostic — not a vibe check, not reassurance, a diagnostic — on whether your October goal time is still supported by the pattern your training has produced.
This post runs that diagnostic. Three signals. A verdict.
If you started your block last week, come back in two weeks — the data is not statistically meaningful yet. The prior post, Fall Marathon Training in Summer Heat: The Two-Phase Plan, is where you should be right now. If you are four to six weeks in, keep reading.
The Diagnostic Moment: Why Weeks 4–6 Are the Right Time to Check¶
One week of data is noise. Ten weeks of data is too late to revise your goal without rebuilding the plan from the ground up. The four-to-six week window is the narrow diagnostic opportunity — enough data to distinguish a pattern from variation, early enough that a goal revision costs you nothing except an updated number.
Here is where the major Chicago training programs stand as of July 15, 2026:
| Plan | Start Date | Program Length | Week as of July 15 |
|---|---|---|---|
| CARA Summer Marathon | June 8 | 18 weeks | Week 6 |
| chi.run Chicago block | June 15 | 18 weeks | Week 5 |
| Generic 16-week plan | June 30 | 16 weeks | Week 3–4 |
If you are a CARA runner, you are six weeks in. If you are on chi.run's block, you are five weeks in. If you picked up a 16-week plan, you are at three to four weeks — on the early edge of the window, but close enough that the signals are readable.
All three groups have enough data. Here is what to do with it.
Signal One — Heat-Adjusted Pace Drift¶
A single run's pace tells you about today's conditions. A multi-week trend in heat-adjusted pace tells you about your aerobic fitness. Those are different things, and confusing them is the most common analytical error in summer marathon training.
The prerequisite: you need to be applying a dew-point correction before reading any pace trend from July runs. The specific conversion — what a 65°F dew point does to pace, and by exactly how much — is in Your Summer Pace Is a Lie: The Dew-Point Adjustment. Apply it before reading any pace trend from this block.
Once you have the corrected pace for each easy run across the last four to six weeks, look at the direction of the trend:
- Stable or improving (within ±5 sec/mile week-over-week): Aerobic base is building as expected. Signal is green.
- Drifting slower (more than 5–8 sec/mile per week after correction): Two possibilities — you are not applying the dew-point correction consistently (the most common error; the calculation requires actual dew point, not air temperature), or aerobic efficiency is genuinely declining. Signal is amber.
- Drifting faster at a rate above 8 sec/mile per week: This looks good. It is not — in weeks 4–6, rapid corrected pace improvement at equivalent effort typically indicates intensity creep on easy runs, not aerobic gains. You are probably running your recovery work too hard.
The drift direction matters more than any single week's number. NavRun's weekly training report plots the corrected easy pace trend across the block automatically, using GPS and weather data from each logged run — the correction runs in the background. If you are building the spreadsheet manually, confirm you are pulling actual dew point from a weather station, not the temperature your watch reports.
Signal Two — Heart Rate Creep on Easy Days¶
Heart rate creep on easy runs — easy-day HR rising week-over-week at equivalent corrected effort — is the clearest early indicator of accumulated fatigue in a summer training block. It appears two to three weeks before performance visibly degrades, which is precisely what makes it useful: it gives you time to act.
Specific thresholds to check:
- Easy-day average HR should be stable or declining week-over-week once heat adaptation completes. Adaptation typically finalizes in weeks 3–5 for trained runners.
- If easy-day average HR rises more than 3–5 bpm week-over-week for two consecutive weeks, after accounting for elevated dew points: that is a red flag. This is not normal heat adaptation — that pattern levels off.
- By week 6, easy-day HR on the same route should not be more than 8–10 bpm above your week-1 baseline at equivalent effort.
The distinction the data requires: HR rising because today's conditions are worse versus HR creeping upward across a three-week trend at equivalent conditions. Compare weeks with similar dew points, not raw weekly averages.
Here is what the creep pattern looks like: a runner whose easy-day average HR was 138 bpm in week 1, 141 bpm in week 3, and 146 bpm in week 5 at similar dew-point conditions. That is not a single bad day. That is the signal.
Three causes of genuine HR creep that are not heat distortion: accumulated fatigue from above-target mileage, running aerobic sessions at a pace that is too aggressive, and sleep or recovery deficits from outside training. All three are real confounders. This post names them and moves on — which one is driving the creep matters for fixing it, but the diagnostic signal is the same regardless. NavRun's deviation alerts flag when easy-day HR exceeds the expected zone for three or more consecutive sessions, surfacing the creep pattern before it compounds.
Signal Three — Actual vs. Target Mileage Ramp Rate¶
The 10% weekly mileage increase guideline is widely cited and consistently misapplied in structured marathon training. The correct frame is not whether any single week's jump exceeded that threshold. It is whether your actual accumulated mileage matches your plan's accumulation curve — and whether any deficit is an isolated event or a compounding pattern.
The 3-week rolling average is the right unit of analysis. A single missed long run due to weather or schedule is not a signal. Consistently running 3–5 miles under weekly target for four to six consecutive weeks is a signal.
Here is the math for a mid-level CARA runner: if your program targets approximately 35 / 37 / 40 / 32 / 42 / 45 miles per week through week 6 (total: ~231 miles, week 4 being the planned recovery week), a runner averaging 90% of target has banked roughly 208 miles. That 23-mile deficit in early base work does not disappear. It shows up as blowup risk in weeks 10–14 when quality volume peaks and long runs extend.
Read your 3-week rolling average against plan:
- Within 8% of plan: No structural concern. Single-week variation is normal, especially with a planned recovery week in the mix.
- 8–12% below plan: Compounding deficit. Identify whether the source is weather, schedule, or fatigue, and whether it is recoverable in the next two to three weeks.
- More than 12% below plan: The deficit is accumulating. A goal time calibrated to the original plan's volume is likely no longer supported by the base you are actually building.
NavRun's weekly training report shows actual vs. planned mileage with a 3-week rolling view and distinguishes between a single missed long run and a pattern of shortfalls — the two interpretations read very differently in terms of goal implications.
Reading All Three Together — The Verdict Framework¶
The three signals are only meaningful in combination. A single amber flag on one signal is not a goal revision trigger. Two or more red flags are.
| Pace Drift | HR Creep | Mileage Deficit | Verdict |
|---|---|---|---|
| Stable or improving | Stable or improving | Less than 8% below plan | Goal stands. Keep executing. |
| Stable | Mild creep (1–3 bpm/wk) | Less than 8% below plan | Amber. Audit recovery. Recheck at week 8. |
| Drifting more than 5 sec/wk | Creep more than 3 bpm/wk | 8–12% below plan | Goal needs revision. |
| Any two signals red | — | — | Revise now. Waiting until September makes it worse. |
If the verdict is revision: the formula is direct. Apply the McMillan or Riegel conversion to your current heat-corrected easy pace to get a predicted marathon time. If your corrected easy pace predicts a 3:55 finish and your registered goal is 3:40, that is a 15-minute gap at week 5–6. Revising to 3:52 and executing the next ten weeks into that target is a recoverable adjustment. Entering weeks 8–14 with overreaching tempo and interval paces calibrated to a goal the base can no longer support is where cycles break down and bodies follow.
Why September is too late. Weeks 8–12 are the quality-load peak of a standard 18-week block — the tempo runs and interval sessions where effort is highest relative to your aerobic threshold. If you enter that stretch with a fitness deficit and a goal time that doesn't match your actual training data, every quality session will be calibrated too fast. Overreaching in quality weeks is where stress fractures and tendon injuries originate. The goal revision costs nothing now. It costs the block if you delay it.
Running This Check in NavRun¶
NavRun's weekly training report surfaces all three signals in one place. There is no spreadsheet to build.
- Open the weekly training report. The Pace Trend chart shows heat-corrected easy pace week-over-week. The dew-point correction is applied automatically using your logged GPS data and weather records from each run's time and location.
- Check the Recovery Signal panel. Easy-day HR trend is plotted against your block baseline — not this week's average in isolation, but the slope across the last four to six weeks.
- Look at the Mileage vs. Plan deviation. The report shows actual vs. target with a 3-week rolling view and flags when the rolling deviation crosses 10% below plan, with enough context to distinguish a single off week from a compounding pattern.
If you do not have a NavRun account, you can run the same check manually — Strava export for the mileage and HR data, a dew-point lookup from Weather Underground for the pace correction, a short spreadsheet for the rolling averages. The report does the same analysis automatically.
Four to six weeks of Chicago training data are now in your Strava history. The numbers are telling you something specific. The diagnostic above is how to read them.
For the dew-point correction math behind Signal One: Your Summer Pace Is a Lie: The Dew-Point Adjustment. For the two-phase block structure that produces this data: Fall Marathon Training in Summer Heat: The Two-Phase Plan.