Riding attempt

Tried to ride this afternoon in the extreme heat (90 degrees and wicked humid). From the start I felt like I had already been riding for 3 hours, all of it uphill. I hoped that the "the hardest part of every ride is the first 20 minutes" rule would apply, but other than an easy flat section that was pretty painless, it seemed like the whole ride was the hardest part. I rode the loop at the wildlife preserve just South of Thatcher Park starting at Ryan Road. I rode the loop once, yelping for bears all the while, then rode the loop to the steep downhill that heads back to the park. Altogether I guess I rode 6-8 miles.
After that I rode 6 miles round trip to Nichol's Market with my son so he could get a snack he has been dying for. Easy ride on the mountain bike.
Anyone have any advice for riding two days in a row without the 2nd day feeling like you can hardly move your legs?
Looks like the rain is coming back starting tomorrow :-(