When I restarted cycling a few years ago I told myself, "Just take it easy. Don't push it."
I started out commuting 3 mi each way to work. It wasn't enough. There's just something about riding. I found myself going for a "quickie" at lunch, then extending my ride home until I was averaging 15 mi per day. I went from a flabby 205 to a fitter 175 lbs. I feel better when I ride than when I don't.
Truth is I don't think about it much nowadays. It's a habit. Get up, dress, hop on the bike and go to work. But for those few days I drive I just don't feel right.