National Geographic asked Olympians for workout advice for weekend warriors who want to improve their endurance, power and speed. Here is the breakdown.For endurance, Rebecca Dussault, cross-country skier, gives us this workout. Jog slowly for 15 minutes as a warm-up. Then, to loosen leg muscles, sprint for ten seconds and follow with a ten-second jog. Repeat. Run at your 5K race pace for two minutes. Jog for two minutes. Repeat the two-minute speed intervals six to...