Mootz's points are spot on. 50 miles on a bike isn't all that hard to do with just a bit of training. Easy for me to say, I bike a fair bit in a year. Start slow and work up to the miles, worry about speed later. If you are riding on roads you won't need a lot of gear, so don't weigh yourself down too much. Eventually your butt gets used to it.

A Brooks bike saddle is worth every penny it costs and will last a lifetime. If you change bikes, keep the saddle.
http://www.brookssaddles.com/brooksengland.html

I own three bikes and all three have a Brooks saddle with springs. If I find a fourth bike I'll buy another one.

You don't need an expensive road bike for a 50 mile ride, a road bike is just faster. You can do this on any bike, mostly you just need to train.

A couple of other things, wear a helmet, bike gloves are a good idea to prevent road shock to your hands and protect your palms if you wipe out, and padded shorts help as well.

These are nice as you wear them under regular shorts.
http://www.rei.com/product/732756?cm_sp=prod*desc_rel_item*element
They provide the padding of bike shorts but you wear them under say nylon cargo shorts so you still have pockets. I started using these last year and really like them.

Good luck, with some training you'll do fine.