I’ve got a 12 hour MTB race in mid-July, and I’m using the 100 mile gravel grinder plan. I’ll be doing as much time out on the trails as possible in the next couple of months, which will mean replacing some of the scheduled sessions with those outdoor workouts.
For longer MTB races like you’re talking about, time on the MTB is essential, as you need to train your upper body and all the little stabilizer muscles to be ready for six hours on the bike. Mental preparation is also important, as you need to be able to stay focused for the entire time.
I really like 6 and 12 hours races. You get plenty of time to dial in the course during the race, and the benefit of your own pit area at the end of each lap. And in most, once you’ve completed one lap, your a legit finisher, so they’re a great entry point for MTB racing.