So I'm going to be a junior in high school next year. This summer, I've been doing base building and increased my mileage to about 60 miles per week. However, I've recently hit a slump. I normally run an easy 8 miles at about 6:45 pace and finish pretty fast, with a nice long run on the weekends. Lately though, when I go out on my runs, my heart rate feels a lot faster than normal, my legs feel heavy, and I feel slow and tired at like mile 4. I get enough sleep, and I think my iron level is good. Any advice would be appreciated, thanks.