Runners are generally aware of the hard easy principle- you reap the rewards of hard workouts by allowing your body to recover and rebuild to a higher level of fitness.
What about recovery? Should you take complete rest and not run at all? Is there any benefit to easy running? How fast or slow should your recovery runs be?
Opinions on this topic vary- check out this interesting thread on the letsrun.com message board.
My two cents on the subject? Here is the post I submitted to letsrun.
Very good discussion here- clearly there are many variations you can apply to the hard easy principle, and it won't be the same for every runner. The key I think may be to evaluate your method of recovery by focusing on your key workouts (intervals, tempo runs, or races). It is there is where you will be able to tell immediately whether or not your recovery runs are too fast or too long. Another twist to consider- not everybody responds to high miles or might be injury prone- if you are one of those then hitting the bike on recovery days after particularly hard workouts is not a bad idea. I think you'll find triathetes in agreement that to some extent cycling can improve running (Ed Eystone has done some decent research on the benefits of cross training). I like to take a mountain bike and find a hill someplace- 2-3 minute climb, and do repeats. Your running muscles get to recover while you can still give your aerobic/anaerobic system a decent workout. I've also heard of ancedotal evidence of spinning classes improving running performance. You can also use other forms of cross training-
You may have read about Lukas Verzbicas- the freshman from Illinois that ran 14:18 for 5K indoors and 8:53 for 2 miles-his primary sport is triathlon.