I like to understand why something works.The article ,at the link, does a good job explaining muscle damage(probably applies to other soft tissue) and the repair process.
It is not the hard run that gets you injured.It is not allowing your body the time to repair and rebuild.Also ez non impact training maintains your cardiovascular system and keeps your mitochondria well fed.This simpe 3 day cycle works well for me at 51:
run hard
non impact xtrn
non impact xtrn
Recently I tried jogging 3 miles instead of the ez xtrning.After a week my legs were so stiff and sore I had to take a week off!
I have experimented with more days between hard runs.Seems like than 3 days just does not work.Less than 2 days off does not work.
The last few weeks I have been doing a 4 day cycle:
hard run
medium run
xtrng
xtrng
1/3 running all hard to
1/2 running
with 1/2 hard and the other half medium
I have reduced days of hard running from 33 percent to 25 percent but have added an equal number of mediums runs.
Bottom line is that running is still fun because I am making sure I give my body the time it needs to make the repairs.