Jameshow
Veteran
I don't think there is any equivalent. Cycling is cycling, running is running, swimming is swimming.
This reminds me of the "how many miles should I ride on the road to have an equivalent ride off road on MTB?" Again, it's not an equivalent.
It's a bit like commuting in town, or riding the open road.
Interestingly I ride a MTB in Devon and it's much more of a workout than the road bike esp with the steep hills you find, which means I get a decent workout, added to which the MTB is great for carp roads!! Some are just covered in flith!!