I had the same problem and swore I wasn't in that bad of shape - turns out I wasn't, it was the bike. I retired my Specialized Stumpjumper and purchased a Scott Scale 29er - what a huge difference! I couldn't believe how much harder I was working than everyone else! It's not necessarily a weight issue, because although the components and frame are a little lighter then my Specialized, the 29 wheels and hydraulic brakes puts it about the same. However, having the 29 wheels makes a HUGE difference in my climbing abilities. It gave me so much confidence that I realized my new potential and started hitting the strength and cross training - now I love climbing