If you're dealing with a running injury, or simply recovering from a big race effort, cross-training can be your friend. Adding non-running aerobic alternatives to your routine allows you to retain cardiovascular fitness by while giving your muscles, ligaments, and bones time to recover.

Some popular cross-training options include:

• Swimming: This low-shock option is great for muscle tone (especially the upper body) and the cardiovascular system, but it doesn't do much for leg speed. If you're new to swimming, make sure there's a lifeguard on duty.  
• Elliptical training: An elliptical trainer is a machine that closely approximates running (or cross-country skiing; some have ski-pole-like options). It allows you to use the running motion without the impact. You can also reverse the stride, like pedaling backward on a stationary bike, which works the muscles in different ways. You might want to bring headphones--elliptical training can get boring--and a towel; sweat doesn't evaporate when you're not moving through the air.
• Deep-water running: This is also very similar to actual running and can be done in NYRR Deep Water Running classes. You can run in the shallow end of a pool (low impact) or in the deep end (no impact). Most deep-water runners wear a flotation belt or vest to stay higher in the water; as you gain experience, you may be able to do without the device. As with swimming, safety first.
• Cycling and/or spinning: Unlike swimming, these low-impact aerobic activities can help your leg speed. You can do interval sessions on a bike or simulate them in a spin class. Using low gears (less resistance) is a great sprinting facsimile, and cycling up a hill (more resistance) is just as tough as running up it. Long rides can even replace your long runs.

With regular cross training, you can maintain your cardiovascular fitness. Most speed workouts, like tempo runs, hills, fartlek, and intervals, can be replicated, too. You might be off the roads, but you're not out of the game.