Mountain Mitch

Mountain bike riding, at least around here, is a lot like doing wind sprints. Road biking is more like long distance running. It will help fitness for MTB but since you have sworn off, I’m not going to try and convince you. MTB involves intense climbing activity followed by less active, more skill oriented descents. You don’t ask about building the skills, just the fitness. I think a good spin program is best. If that isn’t available then build your own with a wind trainer or stationary bike at the gym. But any high intensity interval training is great.

Check around, there may be other, closer trails you can access, even if they aren’t that long or challenging.