It's a lot faster either by yourself or with others of equal gear and conditioning. On pavement, as others have pointed out, anything that fits you will get the job done. Unless you're in really horrible condition, 12-13 mph with a meaningful load on a mountain bike on gravel path (essentially flat) is an easy pace to sustain all day - YMMV, but that's my experience.

If it's a BoB bike, then hardtail mountain bikes rule against the possability of having to go off-pavement. Stay away from softtails for a long ride anyway - unless you really want a work out.

My bike is set up with full toolbag under seat (includes CO2 inflator, several 12gr, spare tube, glueless patch kit, etc etc); Beefiest rear rack available (I had to design & machine a 7075 block to properly mate this setup to my downtube, but that's up to your frame and how you're configuring everything - they usually fit as-is)); expandable top bag on rear rack; easily removable "grocery sack" style panniers; small bag lashed onto handlebars; bottle cage; and I keep a Camelbak Ridgerunner (100oz) on my back. Helmet and padded fingerless gloves a must. Also really good seat (skinny and "correct" for male). Needs a better pump mounted - after I'm sure my boys won't "borrow" it <grin>.

Bike currently has BMX Mosh pedals and I will NOT put clipless pedals or toe clips on it because it has a BO role. The only bad thing about those pedals is that they positively maim your shin if you screw up (once is enough). This lets me wear the most appropriate walking/hiking/whatever footgear for the season/conditions with confidence that they will not slip off the pedals.

This is NOT as efficient a setup as a road bike with clipless pedals running on smooth pavement. I travel slower for equal amount of effort. OTOH, it's positively flying compared to walking.

Highspeed downhill runs are crazy with that sort of set-up, though - if heavily loaded like that, most mountain bikes become very unstable as speeds rise over 30mph (mine sure does!). They have relatively short frames (for agility) and the heavy rear weight bias when loaded like that gets hairy and scary going fast. (So slow down). One thing I would like to do is come up with a bomb-proof no-tool lock-out for my suspension (front), then figure out a quick-mount mount rack and other bags up front. As it is now, I pretty much max out things on the rear when fully loaded for a camping trip.

Anyway, 50 miles on pavement is generally well within most people's grasp with just some common sense and a bike that won't break apart in that distance. Folks on road bikes (with support crews for gear) often do twice that or more in a day in moderate terrain.

My rule for bike trips is: Have fun! I save the grueling stuff for hiking.

/edit Most mtn bikes come with tires that are WAY too knobby for most uses. I have much more reasonable tires on mine and they have a "pavement" rib in the center and Kevlar in the carcass. Much less rolling resistance and frankly, if they slip (never have), I'm in such shmutz that I'm going to walk the bike anyway. One of my boys is a bike nut, er, expert, so I consult with him... his advice has been spot on so far. /end edit

Tom