Quick Answer: If your long run collapses in the final three miles, check these in order: your starting pace was too fast relative to your easy zone, you took in fewer than 30 grams of carbohydrate per hour after the first hour, you started the run already dehydrated, or you have been ramping long-run distance faster than about 10 percent a week. Aerobic fitness is rarely the actual limiter at half-marathon distance.

The fade has a cause, and the cause is almost always earlier in the run than where you felt it.

Why do the last three miles feel disproportionately hard?

Because the cost of everything you did in the first ten arrives all at once.

Running economy degrades as a long run progresses — the same pace requires progressively more oxygen as your form deteriorates, stabilizing muscles fatigue, and stride mechanics shift. Research on this drift consistently shows oxygen cost climbing several percent over an extended effort even when pace stays flat. So the last three miles at the same pace are genuinely harder work than the first three, not just perceived that way.

Layer on glycogen depletion. Muscle glycogen stores hold roughly 90 to 120 minutes of moderate-intensity running for most people. If your long run is 100 minutes and you fueled nothing, you hit the bottom of that tank precisely where the wheels come off.

Then core temperature. Even in early fall, a long run raises core temperature progressively. Above a certain point the body prioritizes cooling over performance, diverting blood to the skin and away from working muscle. That reallocation feels exactly like running out of gas.

All three are cumulative and all three peak at the same time. That is why the fade feels like a cliff rather than a gradual slope. Nothing dramatic happened at mile ten — the bill simply came due.

How fast should the first mile of a long run actually be?

Slower than you want, and probably slower than you have been running it.

The rule of thumb that survives contact with reality: long runs should sit at roughly 60 to 90 seconds per mile slower than your goal half-marathon pace. If you are targeting 9:00 miles, your long run belongs at 10:00 to 10:30.

Most people run their long runs 20 to 40 seconds per mile too fast, which is enough to shift the effort from aerobic base-building into something that accumulates real fatigue. It does not feel too fast in mile one. That is the entire trap.

A more reliable check than pace: you should be able to speak in full sentences for the whole run. Not gasped fragments — actual sentences. If you cannot, you are not in easy zone regardless of what the watch says.

On the first mile specifically: run it deliberately slower than the rest. Cold muscles and fresh legs produce the worst pacing decisions of the entire run, and 30 seconds banked early is 30 seconds you do not pay back with interest at mile eleven.

Heat adjustment matters right now. Running in August humidity raises heart rate at any given pace by several beats. If your long runs have been getting harder over the last month while your training stayed the same, seasonal conditions are the most likely explanation and the correct response is to slow down, not to push through.

What should you actually be eating during the run?

Something, starting earlier than most people start.

The threshold: anything over about 75 minutes benefits from carbohydrate during the run. The commonly cited range is 30 to 60 grams per hour for efforts up to about two and a half hours, which is one gel every 30 to 45 minutes, or the equivalent in chews or sports drink.

Start fueling at 30 to 40 minutes, not when you feel low. By the time you feel it, absorption takes 15 to 20 minutes and you are already behind. Fueling on a schedule is the entire technique.

Practice with the exact product you will use on race day. The gut adapts to carbohydrate intake during exercise, and it adapts specifically. Trying a new gel on race morning is the classic self-inflicted disaster.

Hydration runs alongside it. Aim for roughly 400 to 800ml per hour depending on sweat rate and conditions, with sodium included if you are a heavy or salty sweater. A simple sweat-rate check: weigh yourself before and after a long run without fluid intake, and each pound lost is roughly 450ml you needed.

And the one that gets missed: the run before the run. If yesterday was low-carbohydrate or you skipped a proper dinner, you started this morning’s long run with a partially filled tank. The last three miles are where that shortfall becomes visible.

5 Mistakes That Show Up Only at the End of a Long Run

1. Building distance too fast. The 10 percent per week guideline is imprecise but directionally right. Jumping from 8 miles to 11 in one week is a 37 percent increase, and the last three miles of that 11 are running on legs that have never been asked to do it.

2. Doing every long run on the same flat route. Legs adapted only to one surface and one profile fatigue faster when anything changes. Vary terrain across the training block, not within the same run.

3. Running long the day after a hard session. A tempo run on Saturday and a long run on Sunday is a common schedule, and it means the long run starts with depleted glycogen and fatigued muscle. Put a genuine easy day or a rest day between them.

4. Ignoring cadence as fatigue accumulates. Stride length shortens naturally when you tire, but many runners let cadence drop too, which compounds the loss. A brief conscious return to a quicker turnover in the final miles often stabilizes pace more than trying to push harder.

5. Treating every long run as a test. If you finish every long run at your absolute limit, you are racing weekly, and the accumulated fatigue guarantees the next one starts worse. Most long runs should end with the honest feeling that you could have run two more miles.

FAQ: Long Run Questions, Answered

Should I run through the fade to build mental toughness?

Occasionally, deliberately, and not every week. One run per training block finishing genuinely depleted has value for race-day familiarity. Every week produces accumulated fatigue and, eventually, injury, which builds nothing.

Does the fade mean I will fade at the same point in the race?

Not necessarily. Race day includes a taper, race-day fueling, adrenaline, and a start line rather than mile zero of a training week. Runners routinely hold together in a race far better than in the training run of the same distance. That said, a fade caused by fueling will repeat if you do not change the fueling.

How long should my longest long run be before a half marathon?

Most plans peak somewhere between 10 and 12 miles, two to three weeks before the race. Going beyond 13 in training is generally unnecessary and adds recovery cost without proportional benefit.

TL;DR:

• Long runs belong 60 to 90 seconds per mile slower than goal race pace, and the first mile slower still.
• Take 30 to 60g of carbohydrate per hour, starting at 30 to 40 minutes, not when you feel low.
• Build weekly distance by roughly 10 percent, and never stack a long run behind a hard session.
• Late-summer heat raises the cost of any given pace — slow down rather than push through.
• Most long runs should end feeling like two more miles were available.

The fade at mile ten was decided at mile one. Fix the start and the finish takes care of itself.