Hey guys, I'm a newbie to cycling; I've been doing long (2 hoursish), slower paced rides (14mph) just fine, but as soon as I hit some group rides where the pact is a bit quicker (~19, 20mph) my quads just self-destruct. I can hold my own when drafting but as soon as I'm up front, I get too tired. My bike is a bit on the heavier side, but I figure that's not too detrimental? How would I improve my speed over distance?