Well, I'm just about one year shy of 50, and I got back into cycling a couple of years ago after, um, a few years off (my, doesn't time fly!), so I'll try to make a few observtions.
Set your own goals, but engage others on occasion. If you let others set your goals for you, you'll lose interest quickly. If you never ride with others, you won't push yourself.
Be prepared to see something you never expected to see at our age - physical improvement. When I started serious riding two years ago, I established a 20 mile loop, with moderate grades. It has become my benchmark for progress. My first runs were pretty weak - average speed of 13 mph. Last month, I achieved a landmark for myself - 20.1 mph over that same route.
Once I started seeing improvement, it put the hook in me. I'm getting stronger. As long as I'm seeing improvement, then I'm not decaying.