I really enjoyed this article in the Austin American-Statesman about how much is too much to run. The bottom line is that you can run as much as you want, and the reality is that 99.99% of most people won’t even be able to come close to over doing it. Granted, this assumes you are healthy enough to exercise (i.e. walk) and slowly build up your stamina (i.e. not running 10 miles on your first day out).
This rule applies to just about all forms of exercise, even though there are a few skeptics who have seen a 60 Minutes special on an obsessive person who over did it. Anything under 2 hours of exercise per day is still considered recreational exercise, and squeezing 30 minutes a day is a stretch for most. I recommend alternating between various types of exercise to prevent injuries and increase motivation, but if fear of exercising is preventing a regular visit to the gym or a trip around the block, think again.