Pacing, as Dave said, is key. Here’s a mental-game tip I was given that helps with the mental side of pacing.

There are always three “starts” to a workout. There’s when the clock starts. There’s when you decide to start doing the workout (usually when the clock starts). Then there’s the third start, where your body says “what the hell are you doing to me?” It’s at this third start that you need to remember your pacing plan. If you get to that third start, and you’re smoked, and you’ve still got 10 minutes, time to ease the pace a bit. If, instead, you have lots left in the tank, try to pick it up, but don’t go all out just yet.