First time runner needing help

So I'll start with why I'm here. My father is absolutely amazing. As I write this he is completing a 100 mile marathon at the age of 71. He has been running for the last 24 hours and is still moving. I'm so proud of him. There is a catch though: I made a deal with him that if he finished 50 mi that I would do a 5k, if he finished 75 I would do a 10K. If he completed the race of 100 mi I would do a half marathon.... Well I guess that was all the motivation he needed. So for the help: how the heck do I start preparing for that kind of run considering that I am a 0.0 runner. Luckily I work a fairly physical job where where I'm on my feet walking constantly. But I'm a 40-year-old male that is 241 lb (was 280 but have been losing weight). I still have another 60 lb maybe to lose? How do I start practicing so that I don't completely destroy myself running.

TLDR: Heavyset 40-year-old made a bet to run a half marathon and needs pointers on where to start.