I'm running a 50 mile race (I hear its actually 54) on Saturday, a few days before my 50th birthday. I couldn't have imagined doing this 15 years ago.
Good luck on the 50 Mile! I did my first one earlier this spring and it's going to be an annual thing, despite the promise I made myself the day after to never do another one

I hope the weather cooperates for you!
Bikeknit -- Great advice already! Motivation is overrated. You never hear people talking about how they're just not motivated to brush their teeth -- it's something you just do for your health.
From the squatrx blog (who says it better than I could!):
Squat Rx: "Motivation" is Overrated
Motivation is over-rated. Stop waiting to get motivated. Motivation and enthusiasm will always come and go. Even the best athletes in the world don't always "feel like" training, but they do it anyway. Instead of waiting, start training for something and train
frequently. Do NOT destroy yourself every training session, but do something almost everyday. Make training a habit. By making it a habit, you are virtually eliminating emotion from the decision-making process and, if the habit is ingrained deeply enough, there is no decision to make about training or not - you will train period.
Creating
the right habits, mentally and physically, will help sustain you through the valleys and long plateaus.
Me again: I found one of the most helpful things in the beginning, before it truly was a habit, was to use positive reinforcement on myself.
I use one of the free calendars I get every year and write down what I did. If I'm training for a race, I'll also write in my training schedule. Now, seeing the filled-in days is reward enough, but in the beginning, I'd get myself a little something (usually fitness-related, like a new pair of shorts or yoga DVD) only if I met goals -- like having 24 of the 30 days filled in with activity.
Now, every year I make fitness goals -- for example, run X number of races, win X number of age group awards, run a PR at a certain distance, bench half my body weight, etc. This helps to keep things interesting and challenging.
Good luck!