Cycling is a cardiovascular activity It is not for muscle building. If you just want to go further distances you're really not getting a great workout and that's why your body is becoming fatigued You need to maintain a high cadence. You need to be spinning and get the cardio up. You should still be able to talk but not sing Go for short distances and get the cardio up Going long distances will do nothing for you. Spin the pedals fast, It may seem like you're pedaling so fast but going nowhere, it's OK because you'll get better