Except for me, 75 minutes whomping up and down the hilly gravel roads on my mtb (and that's not even technical, except for loose corners) has me wobbly-kneed for hours, but I have to ride twice as long on my road bike for the same effect. I don't think it's really possible to make a general conversion, except maybe time multiplied by perceived effort, make up your own scale . Depends so much on the weight of your bike, the terrain, how fast and hard you are going.