get a mountain bike. that you can ride on the road, as well as trails. road bikes are, well, road bikes. as far as getting tired, keep riding. push yourself a little further each day. eat breakfast, have smart snacks, and balanced lunch and dinner. also, stay hydrated! also, learn to shift efficiently! this will help you expend energy more effectively. I'd recomend you get a hardtail mountain bike, with a shorter travel fork, possibly with a lockout for road/bike path riding. I have both a MTB and road bikes. I'm more of a roadie these days, but I still love to get my stumpjumper dirty now and then.