+1 on ChiRunning. It literally changed my life. The workshops are probably out of your budget ATM, but there's a fair amount of free resources on their website, and/or the book and/or DVD are more affordable.
One caveat though, they've lightened up some recently, but ChiRunning tends to look at running form as THE solution to any injuries and imbalances. To me, while it's super important to learn NOT to reinforce your imbalances while running, I think targeted exercises along with your running can get you there quicker and more safely.
You know I'm a little obsessed with feet since they've been such a problem for me .... but hips and feet are the two ends of the same movement dysfunction, and if you've got piriformis issues, I think it's likely that some foot work could really help. Even just making sure that my shoes have plenty of room for all parts of my feet, and wearing minimal shoes whenever I can (I don't run minimal) have really helped my feet, along with yoga.
What's helped more than anything I think is keeping in my mind all the time the foot placement cues from yoga, mainly just evenly weighting all four corners of the feet (first and fifth metatarsal heads, and inner and outer heel). Simple cue, LOTS of consciousness and physical work, enormous benefits all the way up the kinetic chain.
And, start slower and shorter than you want to. Also easier said than done. When I first started running it was in a similar situation, a portable workout to do on days I couldn't do my usual workouts, and so I'd go far enough to get the amount of cardio I was used to, and then I couldn't walk for three days afterward. A mile or maybe even half a mile is enough for your first outing. You will want to keep going, but just don't. If you feel good the next day, then try a little farther.
Don't forget to have fun.
Speed comes from what you put behind you. - Judi Ketteler