Bike Training ? Early Season IM

Chose to do IMCDA next year and one of the comments I’ve heard is that I’d have the long rides on the trainer. Which to me begs the question, do all of the long rides “have to be” on a tri/road bike? Why not grab my MTB and hit some ice trails and just do it? Indoor training can be such a beast esp when doing 3 hour spins - as in ugh.