Fuel

Under 60 minutes: water only.
Over 75 minutes: start fuelling. Begin at 30–40 g of carbohydrate per hour from week 5, taking the first one at 30 minutes rather than when you feel empty.
From week 9: build toward 60–90 g/hour on every long run. Use a 2:1 glucose-to-fructose product — glucose transport saturates around 60 g/h, and fructose opens a second pathway that gets you past it.
The gut is trainable and it needs training. Untrained, most people tolerate about 60 g/h. Two weeks of deliberate practice measurably improves absorption and reduces stomach trouble. The best predictor of race-day GI problems is not having rehearsed.
Fluid: 400–800 ml/hour with 300–600 mg sodium per hour, more in heat.
Before long runs: 1–2 g carbohydrate per kg bodyweight two hours out. At 70 kg that’s 70–140 g.
Use exactly what the race hands out, or carry your own for the whole race. Race day is not the day to discover a new gel.

Heat, and where you live

August to October, default to the treadmill at 0.5–1% incline or run before 5:30am. Heat inflates effort at any given pace — the same speed that’s easy in the gym is threshold outdoors in August. Trust the effort and let the pace collapse; the distance still counts. From November, move the long run and all MP work outdoors. Your race isn’t on a treadmill, and pacing on a belt teaches you nothing about pacing yourself.