3. Run a Marathon
This is a true feat of human endurance. It takes a lot of training and only the fittest of people attempt and complete them.
Even if you are not at the pinnacle of fitness, you can still run a marathon. Train as hard as you can, go on long runs coming up to race day. Then when it comes to it, if you need to walk then you should walk. Completing a marathon is something amazing in itself, no matter how long it takes.