Like many a marathoner, a few years ago I decided to sign up for my first 50K ultramarathon. I've now completed three of them. I'm still learning, and I love to share what I'm learning with others.
The ability to complete a 32-mile race isn't based on your ability to run a marathon. The biggest milestone en route to running an ultra comes when you are able to run a 5K comfortably. That's only about 3 miles. Once you can run a 5K, you can run a 10K -- then a 10-miler, then a 13.1-mile half marathon. Once you've established a foundation of health and fitness, you can run a marathon successfully. If you can complete a marathon, chances are you can complete a 50K race.
I'm not saying that everyone should run an ultra or even a 5K race. Find an activity that's comfortable, energizing, and fun. Even low to moderate levels of exercise can improve your health. The goal here has nothing to do with time or distance. It's about health and well-being. Walking is a great place to start. It's also a great place to circle back to time and again.
Fitness is not a destination. It's a fun, relaxing journey. Try it and see where it takes you!