I started running at 35. At that time, I was a bit overweight and couldn't run a mile. Two weeks ago, at the age of 45, I completed my fourth 100 mile trail race. Initially, it was all about fitness and new challenges. It was at some point in training for my first marathon that running became almost integral to my being, and not some kind of "chore". 

Really, you need to find enjoyment in it if you want to maintain it. Gradually work your way up in distance (read up on training plans - there are plenty of them online), challenge yourself by entering races, and if you are running on roads, be sure to give trails a go.