I've been in solid AFIB for more than 20 years, since my mid-40s. Aside from the warfarin and bruising, it hasn't changed my life at all.
I was a runner when it started, and my split times increased suddenly from the 6:45/mile range to about eight minutes. At first the arrhythmia wasn't obvious, and i couldn't figure out why I'd slowed down, but an ECG pinned it down right away. I'd been thinking of getting more serious about cycling anyway, to save my knees, and since I was a little frustrated running so slowly, I made the switch for most of my aero exercise. I've done thousands of rides over the years with my heartrate under pretty good control with digoxin. Max pulse rate is limited to about 120 (or so...I don't bother to check it anymore), and I'm sure my performance is down from what it would be if I were in sinus rhythm, but whole weeks go by when I don't think about, and certainly don't worry about, the fib.
My cardiologist is my age within a couple of weeks, a former marathon runner, now an ultradistance cyclist. He's very supportive and puts no restrictions on my activity, though he does warn me to "listen to your body and stop when things don't feel right." The older I get, the easier it is to find things that don't feel right...