Originally Posted by AzTallRider

Clearly not nutrition for that distance - it's not nearly long enough to deplete your glycogen stores. The issue is likely the duration of your commutes: they just aren't long enough to develop your endurance, so when you do a longer ride, your aerobic system and muscles give out at a point a bit longer than what you have been doing. You need longer rides to develop the endurance you need for longer rides. I faced with a similar situation. I commuted 8 miles each way for a year, but I didn't drop any appreciable amount of weight, nor did I develop what it takes to do longer group rides. I found an alternate long route to work, and also upped the intensity of those commutes. That helped me get to where I could handle the long distances better, to where I could use those long weekend rides to really improve my conditioning. You need long rides to develop/maintain the fitness required do do them.