As well as your body taking a battering, there's the bike too! Bike maintenance time will go through the roof. And if you're already short on time, which you will be training for 24 hour, you're then going to have to cram maintenance in. Road bike is a god send!
Also, MTBing will have a lot of coasting and freewheeling so you'll have to ride longer to get the same workout, and then the climbing effort can be too extreme in terms of exercise when you're trying to build a solid base.
I know it's boring but fairly flat, really long, fairly slow road rides is what you need until you can go for 6 hours+ with ease, it's what I was told and it seems to be working.