Only you can answer that, and only by actually doing it as there are no coversion tables and Way too many variables involved, as you noted. Not only will your fitness level (50 on a mtb is very good) play a role but, also the bikes themselves.
In general, you should see a marked improvement on a road bike vs the mountain bike. They typically have higher gear ratios, are lighter and, by the narrower profile - more aerodynamic.
If I just Had to venture a guess without knowing you or your equipment, I would say "easily 20% farther to start, or the same 50 miles in 10% less time. As you get more familiar with the road bike and it's handling charachteristics, both distance and time should improve dramatically.
Edit: Bill posted as I was typing, I guess. The way I read it was that your 50 miles of MTB was on pavement? If not, then you are B A D. 50 in one day off-road would rattle my teeth out and shake up my eyeballs so bad I couldn't see straight for a week!
Wheelies don't pop themselves. (from a QBP fortune cookie)