Bea

Move the run earlier if you can. The difference between 6am and 8am in July where I live is about 6C and a whole world of sun exposure.

Or run a loop that passes a tap or your car. I do a 5km loop from home on long run days for exactly this reason and it removed the whole problem.

There is nothing wrong with real food for training runs. I use dried figs and salted potatoes on anything over two hours and gels only on race day. Just do not experiment on race morning.

Same, plus wind. Running out into a headwind and back with it makes the second half feel identical at 40 seconds per km quicker.

Also worth asking whether 6:20 is really your easy pace or a formula's opinion of it. Zones from a 10K time are a starting estimate. If 6:00 genuinely lets you hold a conversation and recover fine, the number was wrong, not you. Track how you feel on the next hard session rather than trusting the calculator.

Shoes at 400km with a marathon build coming are worth rotating rather than replacing outright. Two pairs alternating changes the loading slightly on each run and is genuinely easier on calves than one pair everywhere.

The 600km number is a useful way to think about it. I was framing it as one shoe versus two rather than as cost per kilometre.