I ran home from work (10.4 miles) last night in the heat (87 deg). I went very slowly (9:48 min/mi) but it still wiped me out. Ran back in again this morning. It was much cooler (60 at the start, 73 at the end) but still muggy (60-65 dp). The effect of these back-to-back runs, I feel, is like a long run. The last mile of the morning run feels at least as hard as mile 16 of a long run.
I realized this morning that my endurance is really in the tank these days. The marathon was 5 weeks ago and I think it's time to stop messing around. Of course tomorrow I have to take a big fat zero. No way around it.