The simplest way to improve as a runner is to run more, but that can also increase your chances of getting injured. To escape this cycle, many elite runners now spend less time pounding the pavement and more time doing low-impact activities. Members of Alberto Salazar's Nike Oregon Project team, for example, do as much as 20 percent of their training on underwater and antigravity treadmills. You can train on an antigravity treadmill, too – if you've got $25,000 to spare. Or you can spend a tenth of that amount on an ElliptiGo, an outdoor elliptical trainer that rides like a bike and is used by a growing number of athletes, including America's top marathoner Meb Keflezighi, who takes one ride on his ElliptiGo for every run. For an even cheaper low-impact cardio fix, hop on a treadmill, set the incline to 15 percent, and walk at a pace that's fairly comfortable. "Don't wait until you get injured," says Keflezighi. "It's better to cross-train by choice to stay healthy than be forced to do it when you're hurt."