Quote Originally Posted by Crankin View Post
First, assess what you can do to deal with the health issues. Are you on an asthma controller, not just a rescue inhaler? It might be time to visit or revisit your asthma plan. How did your mental health issues affect your ride last year? You might want to think about how you could change that; half the thing about finishing a ride like that is mental. If you are giving yourself negative messages about your riding, you can work on that with cognitive work, meditation, and affirmations. But, those things are called a practice, because, well, you have to practice them.
There are lots of training plans out there, and you probably could start with a plan using your trainer. But, there is no substitute for TITS... time in the saddle. Get some cold weather gear and get outside as soon as you can (March?). Part of planning for any long ride is evaluating your hydration and nutrition. Maybe you need to be hydrating more efficiently all of the time, not just on rides. It makes a huge difference. Experiment with different sports drinks and food. At those temperatures, it would be hard for a lot of people to survive. Read, read, read, and read more.
10-12 hours is a long time to be in the saddle. If you are OK with it, then it's fine. But, if it's a supported ride, find out if there is support for that length of time. You could always shoot for a metric century (62 miles) or a 50 miler.
+1 to all of this.

Did you register for one century or for 500 miles in 5 days? There's a big difference.

10-12 hours is a long time to be doing a bike ride. It certainly can be done, but (assuming the course is open that long -- as Crankin said, you'll want to find about that) you would be one of the last people to finish. So you'll need to be okay with that. I did the Livestrong Ride in Philadelphia 5 times, and every time I saw the same team of guys doing the full century at a relatively slow pace. They knew they would be among the last to cross the finish line that day, but they didn't mind.