Mountain bicycle miles convert to road bicycle miles
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 (5 mph), and others where I average over 20km/h (12.5 mph) 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. 160 km (100 miles, a century) days on roads does my head in more than my muscles, where as 6 hours/60 km (10 km/h or 6.2 mph) 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 (42 km) MTB, stop thinking about it and do it - whats the worst that happens?