Time to distinguish between road bikes and mountain bikes. Road bikes are designed to go as far as possible as fast as possible. The wheel size to gear ratio allows the rider to get the wheels spinning very fast. The narrow profile and forward leaning position minimize aerodynamic drag, while the narrow tires minimize the rolling resistance. The funky handlebars allow the rider to change hand position for long duration riding and lean into an even more forward aerodynamic position.

Road bikes are differentiated into racing bikes (extremely lightweight and incredibly expensive) and touring bikes (with attachments for luggage racks).

http://en.wikipedia.org/wiki/Road_bicycle

Mountain bikes are designed to allow the user to apply as much torque to the wheels as possible, which is good for crossing rough terrain. The large tires ensure good ground contact on a rough, possibly loose surface. The upright position is better position for the rider to produce absolute power, and the wide handlebars provide for better lateral control. Bar ends can provide more hand positions for longer duration riding.

http://en.wikipedia.org/wiki/Mountain_bike

50 miles is a good afternoon workout to some people, but the challenge of a lifetime to others. For people who are in poor shape and not used to riding a bicycle, having a bike properly fit will be big part of preventing repetitive motion injuries.

The saddle is probably first thing you'll run up against. Those thick, padded gel saddles look pretty good when you get started, but if you ride on them over and over again for any length of time, they can start to do damage by distributing the saddle pressure over all the soft tissues. Modern long duration saddles are fairly narrow, hard(ish) and slippery, and probably have an indent down the center for pressure relief. These saddles concentrate pressure on the bony protrusions under the hip bones (the "sit bones") and reduce pressure on the soft tissues. Padded bike shorts provide a bit of padding, while the slick shorts work with the slick saddle to keep you from absorbing heat due to friction. Unfortunately, it's still going to be uncomfortable for a while. Training rides get you used to this.

For more information on general bike riding, here's a good set of articles:

http://bicycling.about.com/od/howtoride/The_Basics_of_Bike_Riding.htm