Recently I rode 26 miles on my mountain bicycle on a dirt/rock mountain trail. How far could I ride on a road bicycle? I was thinking of riding in a 100k and wondering if my fitness from a mountain bike could be translated to a road bike.
As @Daniel said in his comment, use time as he bases for any comparison. I have had MTB rides where I have averaged under 8km/hour, and others where I average over 20km/h over a similar time frame. What does not change is how much effort I put in over that time, but the consistency of that effort does.
What I find difficult transitioning from MTB to Road is on a Road bike you spend a lot more time in the riding position producing the same output - consistency is far more important than on the MTB. The 'mental space' on the road is hard for me as I get bored too easily. 160km days on roads does my head in more than my muscles, where as 6hours/60km technical MTB and I have nothing left in the tank at the end but will still be enjoying it.
If you can comfortably do 26 mile MTB, stop thinking about it and do it -whats the worst that happens?