Weighing yourself before and after a run for a week is a cheap way to see how much you actually lose. Mine was over a litre an hour in July, which explained a lot.
Pat
@pannier_pat
Rides nine miles each way in all weather and has strong feelings about full-length fenders.
One caution: if the drift persists on cool mornings in September, or your resting heart rate is also up by ten, that is worth looking at separately. Heat explains a summer pattern, not a permanent one.
I carry a 500ml flask for a two hour run, so I have been rationing it. Sounds like I need to carry more water rather than fewer gels.
Check your terrain and your route. I had exactly this and it turned out my "flat" loop had a gentle 2 percent descent for the middle three kilometres. Drift in one direction only is usually a hill you cannot feel.
The threshold being so consistent points at fuelling and duration rather than a structural problem. Under two hours you can get away with nothing on a 16km, but by 18 to 22km you are past two hours and running on empty, and tired calves cramp. Try taking something with carbohydrate and sodium from 45 minutes onward, roughly every 30 to 40 minutes after that, on every long run rather than only the very long ones. That single change fixed the identical pattern for me and for two people in my club.
And a clearance trainer at 70 is often a genuinely good shoe from last season, so option two is not automatically worse if you have run in that model before. It is worse if you are guessing.
If you can find a shop that lets you run in them on a treadmill for ten minutes, use that rather than any review. Fit is the entire variable and reviews cannot see your foot.