I did the trans germnay last year so have some experience of the training required.
My question is how much power training to do. Obviously lots of long distance, low HR endurance road work.
Where I live access to long distance MTBing is limited to a a few local courses 7ish Km long.
FWIW. My recommendation is for you to do as much aerobic work as possible ... any kind that is convenient -- including running, swimming, XC skiing, hiking/backpacking, etc.
After that, IMO, climbing is a matter of having the right gearing for your fitness level (a triple, if necessary) + reasonably good riding technique.