I turned 50 a couple of months ago and I've been cycling for the last 18 months and absolutely love it.
I see exercise as a matter of routine. You need to build it into your life. The only exercise I've really done before was almost 20 years ago when I went to the gym 2-3 times a week for over a year. It was simple, plan my nights, have my bag packed, go home change, have a snack, pick up my bag and go. No element of choice involved, no need to decide will I wont I, just do it, and I invariably enjoyed it. The only time I ran into difficulty was if I broke the pattern. If I had a couple of weeks off on holiday it seemed difficult to go back. One day I just never did.
The bike is more complicated. I started as part of a weight loss thing, and it quickly became much more important than the weight loss. I set targets- certain times, a regular mileage, preparing for specific events etc. Above all though I just enjoy it. This morning was a bit of a battle, cold, slightly windy, hard work. Then coming back into town, with a lot of commuter traffic I hit a cycle/bus lane, the legs started to work (ok, maybe the wind I'd been wrestling with earlier was behind me) and it made the whole ride worthwhile.
I've seen it said on here that the most difficult steps you ever take to ride your bike are from your bed to your front door.
A question for you, assuming you're in work. How difficult is it to get to your work in the morning? Why do you manage that and don't manage out on your bike?