I did it when i was 16 on an xc bike with semi slicks, there are two routes across the top of spain, and you can chop and change iirc. We definitely did some small bits of real mountain biking but mostly road and the specially laid out route which is mostly gravel paths and which horses, donkeys, walkers and bikers share.
It is great. One good thing about it is the 'pilgrim's' hostels some are excellent, it means you don't need to carry very much kit at all (no tent, cook stuff, I think I even managed without a sleeping bag).
search the forum though, sure other people here have done more recently than ten years ago.