Yoga is not only good for you, it's essential. Unlike sports such as tennis or climbing, which require a variety of movements, running relies on a limited range of motion. As a result, only a few muscles get trained and tight, and the rest of your body goes along for the ride. If you have any kind of structural imbalance—say you spent years carrying a baby on one hip or you fell off a bike as a kid—it will be exacerbated by running. Effective stretching is the only way to lengthen shortened muscles and keep the body in balance.
For a serious injury, I recommend one-on-one yoga therapy with a qualified instructor. (See the International Association of Yoga Therapists at iayt.org.) Otherwise, an active yoga program—like Ashtanga, power, Vinyasa, or Iyengar—will help tremendously. Start with a beginner's course, and keep in mind, the fitter you are as an athlete, the more difficult yoga will be for you. So take it slow and easy.
Certain poses offer particular benefits to runners. For example, Downward-Facing Dog (Adho Mukha Svanasana) stretches the back and the back of the legs, as well as the shoulder and intercostal muscles, and strengthens the abdominals and arms. Hero pose (Virasana) loosens the quadriceps, instep, knees, and feet. It's an excellent way to reduce the likelihood of foot and ankle injuries, such as plantar fascitis, Achilles tendinitis, and stress fractures. Warrior I (Virabhadrasana) pose stretches and strengthens the quadriceps, opens the groin and hips, and improves balance and concentration. (A modified Warrior I is shown here.)
A word of caution: Never practice yoga in an air-conditioned room. And remember, you don't stretch to warm up; you warm up to stretch. I tell my students, "If yoga seems too difficult at first, it gets easier. If it seems too easy, it gets a lot harder."
—Beryl Bender Birch, founder and director of The Hard & The Soft Yoga Institute (power-yoga.com) and author of Power Yoga (Prion Books, 1995)