David Ertl on the RAGBRAI blog shared some insights about how important it is to use the right gears. He notes that too often we use the large gear in the front so that we can pedal slowly. The problem is that we are exerting too much force on our legs with each stroke and we use more muscle fibers with each stroke. This means that we get tired earlier.
The key is to keep your cadence at over 75 revolutions per minute. You will use lower gears in the front ring, but you will not tire as quickly.
There is a great deal more that you need to know about this before your next ride. Read Ertl's article, Go for a Spin. It is quite informative.
What kind of rider are you? Are you usually above or below 75 RPMs?