Actually this doesn't really hurt. I like these sessions for the off season and maybe they will help you....
1. Aerobic top end
15 min jog warm up 5 x ( 7 min comfortably hard (like a 6 out of 10) 1 min speed walk), cool down...
the goal of this workout is to stretch your aerobic limits. It is just hard enough
15 min jog warm up ( 5 x (1 min steep hill, 1.5 min wall sit, 1 min lunges, walk/jog down hill), (5 x 40 seconds of long bounding)) 15 min cool down
the goal of this workout is to build hill climbing strength and power. It is a good session to keep momentum going if you feel like your body can't handle a high HR run.
Look up Bobby Mcgee. It is hard to write about the drills that should be done but if you don't half ass them, they work.
These three run workouts, in addition to a long run and a bunch of little easy runs have helped me increase my mileage a lot in what seems to be a pretty sustainable way. I am running about 5-6 times a week which I haven't done in forever. Give em a try.