Scary as that is, it's probably not that big a deal. I went into fib 15 years ago, at age 44. At the time I was running 40-60 miles a week, and my times increased literally overnight from 6:30-7 minute miles to 8:30-9. I went through all the tests, tried electrical cardioversion, but wound up solidly in fib, where I've been ever since.
I'd been a casual cyclist anyway, gradually phasing out of running (which I loved) because of sore knees. The slower times (unavoidable) frustrated me, so I switched completely to cycling, where I had no personal records to compare.
You do have to be a little careful with the Coumadin, but it doesn't cause any major problems. I've done everything I want, including several centuries, without any trouble at all. I'm able to keep up with most casual riders my age--I'm certainly not fast, but I wasn't fast before, either, and I'm not the last 50-something person up the hill. This shouldn't wreck your life.