For the last several months I've been averaging about 70-75 miles per week. However, the last several weeks I've been falling apart on easy runs. The breakdown usually happens between 8 and 10. Today I hit the 9 mile mark on my 10-miler in 1:02, fell apart and ended up finishing the run just over 1:10 feeling like my hips had imploded. It was as if my form had completely fallen apart. At no time during this run did I feel like I was pushing hard. As soon as I finished, I walked about a block to loosen up and felt great again like nothing had happened.
This same phenomenon occurs during tempo runs. When I do intervals though (i.e. 4xmile at 10k pace with 400 meter jog), I don't have any problems.
I've tried correcting the problem by stretching my hip flexors, hamstrings, quads and glutes, as well as taking time off. Nothing seems to help. Has anyone else had this problem?