"It's an aerobic sport, dammit!"
The particular nuances to xc mtb, even in marathon distance, still make this a "simple" premise. A high watt/kg is going to be your friend.
Now with mtb as you know sometimes the manner in which the power is delivered is highly variable. Even in a dead-flat TT there will be peaks and troughs and spikes all over. Mtb is even more exaggerated.
So what does this mean for your training?
In short you need a massive (FTP) engine being able to deliver and withstand repeated supra-threshold efforts over a long period. Some people may fall into the trap of trying to improve say, their 30sec power. Whilst in mtb there are repeated anaerobic range efforts the higher your lactate threshold the less you have to "dip into" this energy system and will also mean better recovery. That is not to say you don't do 30sec efforts, rather they would be well below your 30sec max and would be repeated many times.
The principles of specificity reign and with a discipline like mtb this is particularly true due to the larger skill component and the fatigue experienced by the rest of the body which would be not as notable in road racing/TT etc.
So unsurprisingly most of the work should be done on mtb on terrain which tests your skills. I am also in favour of doing longer sustained efforts as you would for road racing. That being said if you can do said sustained efforts on the mtb (long dirt roads?) even better. (I for one quite like using the mtb on normal roads and occasionally cutting into some single track/dirt.)
If actually getting out on the mtb is an issue then there are no great issues with doing the bulk of the training on the road bike. However due to the aforementioned fatigue on the rest of the body this is perhaps one of the rare occasions I would advocate maybe doing some exercises beyond just cycling (namely pushups, pull ups, planks and isometric holds). If you're doing any sort of decent rides on the mtb this will be unnecessary.
So a sample week may look something like this (depending on time available):-
Day 1: 3hr+ Steady state ride (zone 2)
Day 2: 2hr mtb single track (5x climb/descend sections)
Day 3: 1.5hr road bike with 8x 30sec effort/30 sec recovery, 30mins recovery, repeat efforts.
Day 4: 2hr mtb single track.
Day 5: 1 hr mtb very hard effort, recovery, skill session.
Day 6: 3hr easy ride + 2-3 hard climbs (20mins in duration).
That is just an example of a week I've done in the past. Another consideration, especially if the weather is foul, is doing "micro-intervals" on the trainer. The way these can be composed are legion but things like [5sec On, 5 sec OFF], [8sec ON, 12 sec OFF] etc. The manner in which these intervals are done is that say your 10min power is 300watts then doing 10mins of these intervals should give you an average of 300 watts.
Hope that helps some.