Thanks so much for the writeup @nichelles. Would you mind posting your story on it's own thread? I think your story is really interesting and just don't want it to get lost in the mix.
I had not heard of pericarditis before. Has the medication helped you make any progress? Have they said how long it will take to see the symptoms subside? I'm also curious if they think the symptoms you are feeling are from the MB or the pericarditis?
I'm really hoping you can conquer that and get back to running. If you were not having MB symptoms before all this, I don't see why you wouldn't be able to get back to it. The key here is taking things slow, in moderation and listening to your body (and your doctors). Getting told that doing what you love may result in sudden death really sucks. I know how you feel there. I've been through the same thing with my running. I still run a lot and have done ultramarathons since my diagnosis, just not as far or as fast as I used to go. One suggestion I'd have, if you were a road runner before, try getting out on the trails. Gravity is a nice equalizer. It's ok to hike up the mountain, go slow, then running down is easy! You can keep a low HR while running downhill and have a ton of fun. Plus in trail running, no one cares about your times, you're not pushing any pace, it's just about being outside and enjoying nature.
I agree on the weight lifting and swimming ideas. Both are great low HR activities. I've recently got back into lifting also, it's amazing how good of a workout I can get in while keeping a low HR the entire time. I'd also suggest cycling, tons of fun while keeping a lower HR and impact than running.
Don't say [ex]-athlete! You're going to conquer this and find a way to do the things you love while listening to your body and being careful. There is a way!
edit: btw, the advice you'll likely receive regarding activities will be to keep your heart rate below 80% of your max. If you don't know your max, you can get a ballpark number with the equation: 220 - [your age]