That sounds a well hard event.
Seems odd you wouldn't train on a roadbike (outdoors) though?
I trained only once for a mountain bike race; I did my heart-rate interval training on my road bike because of a nearby long hill with little side traffic so that I didn’t get any interruptions and could maintain a steady effort during each interval. I suppose my eBike with the motor off would have been even better because the slower speed would make the hill seem longer.