This happened to me last year. Unfortunately a 6:40 mile pace for easy runs isn\'t as easy when you\'re running in hotter temperatures and higher mileage. The key over the summer, especially in the south, is to keep mileage high and the pace SLOW. Last year I kept building mileage and never slowed my easy pace. Since I was living in Florida with temps approaching 100, I felt more and more fatigued each run. When I got to XC season I was burnt out with the original goal of my summer--to get faster-- failing. I had to watch my teammates who peaked at 45 mpw over the summer beat me in 5k when I was peaking at 70 over the summer.
Know your limits and slow down significantly for a while. You should also forget your tempo efforts for a while. Once you\'re out of this rut, you can add your tempos back but keep your easy runs easy.
PS- I now measure my heart rate to accurately tell my easy run pace for different tempuratures.