see if you can borrow one from any of your mates, honestly, if you haven't done it before you may be in for a shock.
what's your average speed on a MTB ride. 6-8mph over a 12 mile route? now how long is it going to take you to do 32+ miles?
I'm slow as said before, I managed 11-12mph on a hilly route on the road bike, depending on headwinds.
Let's say you've got a route that would make Holland look mountainous - average 14mph on a road bike over that kind of distance & you're looking at 2hrs 20+ - manageable.
Add knobbly tyres & they'll slow you down to say 10mph at a conservative estimate - and there's a lot of drag there - 3 hours plus. Still manageable?
Try before you buy. You can get a reasonable road bike for under £400. You'll be surprised how quickly you can get the money back by not spending it on petrol...