The Math of “Time in the Bank”
Lots of runners try to bank time early, for example by running 10 sec/km faster than goal pace and hoping to hang on. The problem is that the math is brutal. A surprisingly small slowdown later can erase all your early gains.
This article is not trying to predict your exact fade. It gives you a simple break-even model to show why aggressive early pacing is usually fragile. At Marathon Pace KM, the goal is practical pacing judgment: use the math to pressure-test your race plan before race day, then combine it with effort, fueling, and course conditions.
This also connects to threshold-based marathon preparation. Runners using Norwegian Singles for marathon training often benefit because controlled threshold work can improve pace judgment, but the race itself still rewards restraint, not optimistic arithmetic.
The one equation that kills “banking time”
Suppose you run the first D km Δ seconds per km faster than goal pace.
You gain Δ × D seconds early. If you then slow by S seconds per km for the remaining distance,
you lose S × (42.195 − D) seconds.
Break-even slowdown
The slowdown that wipes out your early gains is:
Break-even S (sec/km) = (Δ × D) / (42.195 − D)If your actual slowdown later is bigger than this, banking time makes you slower overall.
Calculator: how fragile is your banked time?
Try it: keep 10 sec/km fast for 10 km. The break-even slowdown is only about 3.1 sec/km. In real marathons, late-race slowdowns can easily be larger than that when runners overpace or underfuel.
Break-even table (Δ = 10 sec/km)
How much can you slow later before your early banked time disappears?
| Early fast distance (km) | Break-even later slowdown (sec/km) | Interpretation |
|---|---|---|
| 5 | ~1.3 | Slow just a tiny bit later and you’re net slower. |
| 10 | ~3.1 | If you slow more than 3 sec/km later, you lose time overall. |
| 15 | ~5.5 | Even modest fade wipes out the early gain. |
| 21.1 (half) | ~10.0 | You must run the second half very well to benefit. |
| 30 | ~24.6 | By 30K the damage is often already done if this was unsustainable. |
| 35 | ~48.6 | If you can hold 10 sec/km fast to 35K, your original goal was likely too conservative. |
What real marathon data says about pacing (and the wall)
- Large-scale analyses consistently find that faster runners pace more evenly than slower runners.
- Late-race slowdown is common in recreational marathons, especially after aggressive early pacing.
- Physiological modeling helps explain why small early pacing errors can magnify glycogen and fatigue problems later.
- Systematic reviews of marathon pacing strategies generally support controlled starts and steadier pacing over optimistic time-banking.
References
- Ristanović L, et al. (2023). The pacing differences in performance levels of marathon and half-marathon runners. Open access
- Smyth B. (2021). How recreational marathon runners hit the wall: A large-scale data analysis of late-race pacing collapse. PLOS ONE (also PMC)
- Rapoport BI. (2010). Metabolic Factors Limiting Performance in Marathon Runners. Open access
- Sha J, et al. (2024). Pacing strategies in marathons: A systematic review. PubMed
3 simple rules to avoid the 30–35K blow-up
- Start controlled for 3–5K. Your HR and RPE should feel almost too easy. You can’t win the race here — you can only make it harder later.
- Lock into goal effort, not ego. If conditions are tough (heat, wind, hills), pace by effort and accept the time adjustment.
- Earn the right to speed up after 30K. If you’ve fueled well and your effort is steady, then gradually lift pace.
This is also one reason some runners do better with controlled threshold support in training: Norwegian Singles vs tempo runs. But if threshold structure tends to make you push too hard rather than judge effort better, read who should not do Norwegian Singles.
Related guides
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Notes: This post uses a simple arithmetic break-even model plus pacing research. It is not claiming to predict your exact fade — it shows how small fades can erase early banked time.