I have had first degree AV block for decades, since the first time anyone did an EKG on me in college. What it means--in most cases--is that the time interval in the first part of the heartbeat sequence is just a tad longer than normal, but--and this is important--every single electrical impulse gets through. There are no missed beats due to first degree block. So it is almost always considered a benign condition, unless the heart block itself has been caused by heart attack or other disease (which is unlikely if you are young, as I suspect you are).
Before this diagnosis, I was a gym rat, and I have remained a gym rat. I go to the gym five or six days a week and I work out hard. I also have periodic episodes of PVCs, sometimes lasting for months, and I still work out on the same schedule. When the PVC attacks arrive, they make me nervous in the extreme, but I have found ways to deal with that kind of fear, and I continue exercising vigorously.
I'd recommend you go back to your doc or some doc and ask for 'permission' to run or work out. My guess is that after a checkup, you will be given clearance and told to live your life to the fullest, without letting fear get the best of you.
