From one new person to another, here's what helped me the most (others have said this already):
Pedal fast in an easy gear. You'll last MUCH longer.
Make sure your bike is comfortable. All the generalizations for calculating fit are estimates. Listen to your body to tell you what's comfortable.
Pedal in circles instead of just pushing down. You'll use more/different muscles instead of taxing just your quads.
Take hills slow, and don't burn yourself out. Pam, another member here, gave me the excellent advice to use the hills for resting. OMG she was right. For steep ones, it is totally OK to ride at the pace of a walk.