Progressive Run

I met up with Sean, Luis and Bill at the park this morning for a progressive run. They were going to run 7:00, 6:45, 6:30, 6:15, 6:00, 5:45, 5:30, 5:15, 5:00 and I was going to hang as long as possible. I thought I would be able to stay on pace through 5:30 and then see what happened on the 5:15 mile. It was 77 degrees and 90% humidity when we started. I had run an easy warm up mile to get going before we met up with the group. I felt pretty good the first 5 miles, but I started struggling in the 5:45 mile. I didn't drop off pace, but I was really struggling and I knew I wouldn't be able to hang on the 5:30 mile which I thought I should have been able to do. Sean had told me that before I dropped out I needed to come up to the front and lead for 1 minute. I pulled up to the front near the end of the 5:45 mile and ran about .1 miles into the 5:30 mile before dropping out. My actual splits were 6:56, 6:42, 6:30, 6:14, 5:59, 5:40.

It was a tough workout, but I feel like I should have been able to hold on longer. I've done tempo runs where I get down into the low 5:40's for several miles so I felt like I should have been able to do better than a 5:40 finishing mile. I know the weather conditions were bad, but I was kind of embarassed at how early I dropped out because I was sure I had at least another mile in me before we started. I wonder if I'm running too hard on my easy days. I'm running ~7:00 pace on a lot of runs, but in 90+ weather that is a much harder effort than doing it in 70 degree weather. I don't have any trouble acclimating to the hot weather when I am just cranking out mileage and I can maintain a pretty decent pace without falling apart, but the heat and humidity has just been kicking my butt on the hard workouts and races. I'm not sure what I need to do to acclimate better to running hard in bad weather but it seems like I am tanking a lot worse than a lot of the other guys in the heat.