Heat does me in. I can do three nine-minute-miles in 80 degrees and moderate humidity. Increase that to 90, in the sun, and higher humidity, and I can do two 11-minute-miles.
I'm doing E25K, which is a little different (schedule in the link in my sig), but [snip]
I tried this past Monday and barely ran 10 minutes total, probably due to the 100° heat, and haven't run again since this week
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