Hi Squeeze,
This sounds like a great mental and physical challenge. I've never done an endurance event of this sort. From what I've read, it is important to build up your base mileage through short and long walks and to have a strict plan for when (and what) to eat & drink and when to rest. This article has some interesting info on (among other things) the food aspect of a race such as this:
http://www.centurions.org.au/train.shtmlI have trained up for fights before, and while the training and timing might be a bit different, I think that the concept of peaking and tapering applies to what you are preparing for. Peaking will help you avoid overtraining before the event and tapering will help you go into the event fully rested.
As for your specific question re: exercises, three exercises that I think you might find helpful are:
1) Walking with a weight vest.
2) Low-weight/ high rep squats (i.e., air squats done with a weight vest or just bodyweight). You could try modifying a MURPH and do a 1 mile run/ 300 air squats/ 1 mile run for time.
3) Single leg medicine ball toss (this is a partner drill) and standing with one leg on a balance disc to strengthen your ankles.
This video has some exercises meant to strengthen feet and ankles:
https://www.runnersworld.com/training-v ... e-workoutsGood luck, and keep us posted on your event!
Best,
Greg