Pace

Another key to success--think: the tortoise and the hare, albeit perhaps you can find a reasonable speed in between those two extremes. Slow down (note: that is most likely needed) to a speed that you can consistently maintain. With caution, ever so slightly increase the speed and watch as you build endurance.

Before you know it, you'll be stronger than you were. Because you developed the strength over time, it will remain with you that much longer--similar to a habit.