I've found it helps to actually not overdo it.
When I started running again and was fairly out of shape, I did this 10 minute thing. The second or third day I felt great, figured I'd run longer. Next day I was so beat that I didn't go out at all. And 10 minutes every day is better than 20 minutes and then nothing at all.
I think that in the beginning regularity is paramount. This helps build the habit, which is the kind of "discipline" that actually helps you get out there, as opposed to "will power to force yourself to move", which may fail for random reasons (feeling under the weather, being randomly tired, wanting to finish just this one thing, etc).