When you are pedaling hard, the suspension will bob up and down. That motion is energy that isn't getting transfered through the pedals to the rear wheel.
50 mile rides aren't going to be EASY for awhile unless you are already a long distance runner (I'm guessing not in this forum). It will take time for your body to get comfortable on the bike . Any bike that you are comfortable on is a good distance bike. Take a look at what the guys in the long distance riding and touring sections of the forum ride.