Don't lots of you distance guys train in the morning anyway, because it's too hot the rest of the day? The competitors might actually be more comfortable in the morning, no? But I agree, if you're going to move it to 10am, then move it to even 8am--huge difference.
High temp is much less of an issue IMO than medium-high temp + humidity.
Worked out day before yesterday, it was 90 with some humidity, but the hyper-local humidity was off the charts. I was doing 4x100 repeats with 50m walk, on a soccer field that had recently been irrigated, at 3pm. It was almost steaming! Reminded me of Toronto, Philly, and DC, only way worse. Was sweating my bag off just standing in the shade doing some stretching.
I love severe conditions, but man was I bagged after this one. Went home and basically crashed. Really sucked the life out of me.