What factors make you feel like you can't do any more? Plain ol' boredom? Muscles? Aerobic fatigue? Pain in the butt? What makes you tired? Answer that question and then coddle that aspect of your riding. If it's pain in the butt, then going faster will help that a lot 'cause you'll prob'ly be leaning forward more and of course you'll simply have less time *on* your butt.
That's the part I know about, 'cause that's the part that gets tired for me, but I'll hypothesize about the rest... If it's muscles, you want to make srue you're using good technique that uses lots of muscles all the way around the stroke of the pedal, and listen to muscles getting tired, and be sure to eat and drink right and keep those electrolytes in balance.
Are you a "keep going the same pace, don't stop" rider, or do breaks every once in a while really re-energize you? Recognize your pattern and work with it.