MTB rides on road plan?

Hi - I was just browsing various plans including mountainous Fondo (med vol/2:1)and noticed that the weekend outdoor sessions are sometimes intervals on a mountain bike! Is that right?
I also noticed that all the plans I looked at are based on indoor only. Sure you can take some outside but I’d prefer a plan that “told” me to ride outside…(need to train my self discipline!!)
(browsing for sthg that would be good for a multi day mountainous non-timed challenge next summer)