I've done a couple ultras.... don't kill yourself over HN-cred.
Edit: let me elaborate. In my experience and many others' it takes much longer than 3 months to train for a 50k, let alone a 50mi race if you hope to finish, even if you are able to handle a 12k and nature hikes. I ran 125mi through the Sahara a few years ago as part of a stage race and even after 9 months of training I still felt like I needed more training. Don't buy into the snake oil.
I second that. Don't buy the snake oil. People get killed running half-marathons that didn't train properly for.
I know people who seem to be able to run full marathons without any training. It varies from person to person. If I was stupid enough to do the same, I'd have a coronary.
Edit: let me elaborate. In my experience and many others' it takes much longer than 3 months to train for a 50k, let alone a 50mi race if you hope to finish, even if you are able to handle a 12k and nature hikes. I ran 125mi through the Sahara a few years ago as part of a stage race and even after 9 months of training I still felt like I needed more training. Don't buy into the snake oil.