Yoga is a fantastic tool for runners, as it can provide wonderful support for strength and flexibility, especially in muscles you tend to ignore in training (like your stabilizing muscles).

However, it's incredibly important to approach yoga cautiously and conservatively if you're new to it (OP is not new, but other people reading the thread could be). You have to recognize your own limits and work within them, otherwise you'll only increase your risk of injury. A good teacher will help with this, but if you're new to yoga, it's easy to overextend yourself too quickly.

Like anything, ease into yoga, be mindful of your body, don't try to compete with anyone, and give your body ample time to rest. If you do all those things, you should be great :D

P.S. If you're having pain, you should really see a doctor or physical therapist. Yeah, it sucks to consider the possibility of ruining your training or racing plans if the injury is bad, but not treating it will only make it worse for you later on. I learned this the hard way when I was reluctant to see someone about a calf injury; had I just bitten the bullet and seen someone right away, I probably would be fully recovered a month+ early, rather than having to endure a more weeks of recovery and -no- running.