Hi Mark! I’ll start by saying that I haven’t ridden 50 miles in a day on a unicycle yet, and I’m interested in seeing what advice others have to offer as well.
I know from doing long-distance running and bicycle rides that the most important preparation is doing a big volume of distance at an easy pace, and that the proper easy pace is easier than what most people tend to do when they think they’re going easy. Get in the miles and get in the time in the saddle. You don’t need to learn any tricks or special techniques, you just need to get steadier and more relaxed and to build endurance and get experience dealing with things that happen around you when you’re out on the road with bikes around you.
Comfort is everything on long unicycle rides because the unicycle grade of uncomfortable is serious pain. While you’re training, you should be figuring out what combination of saddle and pants works for you, and whether you want handlebars and what kind if so. Also, pedal and shoe comfort is something that turns out to be an issue for me as I’ve gotten up to a couple of hours unicycling. Don’t expect the first thing you try to be satisfactory for any of those.
Based on charity bike rides in my area, 10 mph average would probably let you to stay with the tail end of the group, and that’s a pace I can keep up for a good while on my basic ungeared 36". The bicyclists back there might go a bit faster than you when they’re going, but they tend to take breaks and dawdle around a lot and you could match their average if you were steadier.
If you go with a 36", I think you’d find that it works well enough off pavement too. The tires are 2 1/4" wide and unicyclists have done plenty of serious mountain trail riding on them. Of course there are trails and then there are trails, so it depends on exactly what you’re talking about, but on a well graded unpaved path I think you’d be fine.
Quick brain dump… Let’s see what everyone else has to say. And good luck with it!