That sounds like a lot of fun. The hardest thing will be recovering day after day. You'll need a big base for that. If you are looking to place well, you will also need some speed work for the special stages. If you're "just" looking to finish, then you can pretty much skip the speed work. I'd still do some SST like climbing efforts though, they seem to be good training for maintaining a good pace for a long day of climbing.

One thing that helped me for the everest challenge, a hard two day stage race with lots of climbing, is to do two hard days with a lot of climbing in a row. The second one sucks, but doing that on a lot of weekends got me used to it for the race. Since you're not working, you can go for more long days in a row when you're ready for them.

Many serious MTB racers in the US train on road bikes, you might consider that if you don't have one already and it is possible for you.

It sounds cool, keep us informed on how you're doing!