Six years ago, I got the diagnosis of cervical osteoarthritis as my 39th birthday present (the appointment was on my birthday, lucky me), and the physical therapist advised me to stop all impact activities. So being the good little trooper I am, I stopped running and tried to pursue other aerobic activities. Tried the elliptical; didn't do anything for my heart rate. Took up spinning; that just made my thighs even huger. Became an expert at racewalking; now THAT I just totally hated, because not only did I look stupid doing it all through the neighborhood, it felt like I was going so slow that I was traveling backwards.... needless to say, the following months were sheer **** for me. I missed running, and due to that, plus other reasons, I fell into a depressed funk. I got the diagnosis in May. By the following March my husband couldn't stand me anymore, and told me to risk hurting my neck and start running again (he said it and meant it a lot nicer than it sounds

). So I went against doctor's orders and started running again.
So what happened? NOTHING. My neck didn't break. I wasn't debilitated and crippled. I ran. I worked on improving my upper body posture while running from what I had learned in pilates and yoga, and NOTHING BAD HAPPENED. Granted, I was older than you, and my arthritis is in a different part of the body, but still, take everyone else's really great advice here and get other opinions. See medical professionals that are runners. Take up yoga to learn how to carry yourself in order to minimize your symptoms worsening. Or if you have no symptoms yet, to keep them from occurring. Most important, don't give up what you love, because otherwise, what you save won't be worth saving.
Oh, and for what it's worth, since that time I've also been told I have arthritis in my foot and upper back. Probably have it in my shoulder too. I still run. My S/I is totally wonky at times, and both the chiropractor I used to see and the physical therapist I see now both agree that my running helps keep me level. It's my sanity, and I wouldn't give it up for anything.
Good luck!
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....still breathing...
E.