I started off mountain biking in the early 90s and if we wanted to ride in the Peak district we had to ride out there from Manchester, do the ride then ride home again, so we were pretty fit, racing all over the UK, did the Grundig at Strathpeffer…
Then later I decided to road race and got hammered, got hammered in the training rides, got hammered in the chain gangs and got hammered in the races.
Then I got to be ok in the road races, then went road racing in Belgium and got bloody hammered again.
I would say, in my experience, road racing is way harder than mountain biking, and even more so these days as mountain bikes get ever more specialised so actually get ridden over fewer miles than back in the day.
Having said that, mountain biking can be as hard as you can make it, if you choose to make it hard..