Ignoring Weights
Record-holding American distance runner Ryan Hall lifts weights. Michael Phelps swears by strength exercises. And in a recent study by the Norwegian University of Science and Technology, a group of competitive amateur athletes ran 21 percent longer at their maximum aerobic speed after doing half-squats for eight weeks. Translation: When done right, strength training improves efficiency and performance in endurance sports. Ignore weights and you won't reach your peak.

The Fix: Add two resistance-training sessions to your weekly calendar. Focus on free-weight, multi-joint movements, like one-arm dumbbell snatches: Place a dumbbell on the floor between your legs, bend your knees and flex your hips, grab the dumbbell in one hand, bring it up to your chest, and lift it straight overhead. "Early on, do more repetitions and lighter loads," says Eric Cressey, author of Maximum Strength. And don't worry about bulking up: As long as you do enough aerobic exercise, you won't turn into a meathead.