I live in Texas so that means when summer comes around i have to deal with hot and humid days. Last summer i upped my mileage from about 35 to 60 miles, running many miles in the 720-740 pace range in the inescapable humidity of the day. Within three weeks of slightly raising my mileage I noticed a considerable jump in fitness when doing a tempo. By the end of the summer i had reached 60 mpw and was stronger than i have ever been.
Fast-forward to December after XC has finished up and i'm building up a little base for track. I run about 5 weeks of 60 miles a week, trying to get up to the fitness i had during the summer. I didn't reach the fitness that i had reached within the first 3 weeks of summer running only about 50 miles a week! (Note: my easy pace naturally dropped to around 7 min. a mile) I ran similar workouts that i had during the summer, and a little bit more mileage, and i felt worse!
By the end of that period i decided that i needed to try something or i may suffer through track, so i began touching marathon pace much more on my runs which helped.
What i'm getting at here is, has anybody else had a similar experience? Is running in humidity an added stimulus? Is it comparable to the effects of altitude training?