Need Some Help?

if its a mountain bike its probly workin you harder. Fat tyres = more rolling resistance. Heavy bike = harder to get up hills. Giant, Trek, Avanti are some good brands with low price models. also, at this time of year bike shops are trying to get rid of last years models so nows the time to buy.

I was in almost the same position as you, I ran at the various levels of school cross country and had a pretty standard bike. After an absolute flogging at state titles and several people I already knew I decided to save up for a road bike. Believe me you will love the change to a road bike, it feels easier while doing more for you physically than a normal bike. If you really have your eyes set on a paricular bike at your LBS or something negotiate with the owners and if you are saving the cash try to get to pay it off on lay-by. That way you know what you are getting, you cant wait to get it and you will actually get it instead of I might get that or I might get that like I did for several years. You will love a road bike and it will do you a world of good.
Good luck with your running and cycling.