Also, make sure your bike is adjusted properly. When I first bought a bike I bought one on sale that was a little small for me... the girl who sold it to me mentioned it but I was the next size up wasn't on sale so I went with it. A few days later I figured if I'm riding as much as I planned to I better get a bike that fit me. I went back to exchange it and wanted to go through the fitting process but it there were different people there who said it didn't matter, it was all about "comfort". Honestly I hadn't been cycling in over 10 years so I didn't have much of a basis for what was comfortable. I figured I could ride it without feeling awkward and it was fine. Meanwhile my 4.5 mile ride home from the light rail station, which featured some fairly heavy climbing, was kicking my behind and I figured I was out of shape. After about three weeks it was still killing me and I was really frustrated that I my conditioning wasn't improving. Finally a friend mentioned that the saddle height should be adjusted so you can fully extend your leg on the pedal without shoes, and have it slightly bent with shoes. I adjusted it and immediately it made a huge difference. No more aching knees or fatigued legs. Thinking about it, it makes sense you should be able to get more of your leg into the pedaling stroke... not doing so puts more strain on your knees. Now I'm not even considering taking my bike the light rail and riding 11-14 miles each way on a regular basis.