I used to ride about 200 miles per week. Like Mootz, I began riding a mountain bike, then switched to a road bike. Still own both, as well as a 1970's muscle bike with a banana seat. If given the choice, I'd ride a road bike for any distance over a mountain bike, but 50 is doable on a mountain bike. You're going to put in about 150% of the effort someone on a road bike puts in. A cyclocross bike is somewhat of a middle ground between them, and you could put tires close to a road bike's on one.

I would however were bike shorts and nothing else. I'd go buy a decent pair with real chamois in the crotch and not worry about showing off my wares or any camel toe. If you chafe during the ride in a regular pair of shorts, you might be miserable. Bike shorts should go far in preventing chafing.