Well if you are tired after riding a road bike for 50 miles and you recover quickly and can go out for another ride later that day or the next day, then you probably just need some more miles behind you. A training program can help as to will a coach or someone that can see what you are doing each time you go for a ride. You aren't wearing your self out trying to push a big gear (high ratio gear) all the time are you?
If after doing 50 miles you are tired and wore out for days, then you probably are getting dehydrated from not drinking enough fluids while riding.