I'm a runner, I know runners with AFIB. Though I don't have afib I've been around it a lot (I have a different condition with PVCs). Everyone is very different in this regard.
Sounds like he had Lone AFIB. Cardioversion cannot fix the root cause of the afib. The shock just hopefully stops the chaotic electrical pattern. It resets it. It's the first step to finding out what's wrong. If it doesn't ever come back, awesome! If it comes back there's an electrical problem that is usually dealt with by trying to find it and burn away the pathways.
The ablation has a better shot at fixing the root cause but these don't have 100% cure rates. more like 70-80%, but even that depends on the type of ablation (there are different techniques), such as pulmonary vein isolation, or 5 box mini maze. and there are also meds they try to use.
Hyper-runners have a much higher risk of developing AFIB. Not sure if your husband hyper-trains, meaning he runs more than 10 miles per week or competes in a marathons. If someone, even an elite athlete, vigorously trains >60 minutes, micro damage starts to occur that becomes permanent and you end up with CAD and AFIB after years of that. Anything greater than 15 miles per week, the benefits of running starts to go away.
AFIB won't kill him. If he goes back into AFIB his heart will still pump OK though output will decrease, the problem is with the risk of stroke. The more times you go into AFIB the higher the risk of it becoming chronic meaning cardioversion won't convert it back, and ablation can't fix it. Once you are in chronic AFIB you have to be on blood thinners to reduce the risk of stroke.
Here's a great video from a cardiologist who was an avid runner.
https://www.youtube.com/watch?v=Y6U728AZnV0
Copy the link above into your address bar and check out this video, good stuff.
I think the cardiologist and he is doing the right thing to get back into physical fitness and challenge it. If he goes back into AFIB they can tune their approach to the ablation. If your husband is over-training, then his risk for ending up with chronic AFIB will go up. It's really going to be a trial-error thing and lifestyle modification to address it.
Sounds like he had a successful procedure and your fear of him having SCD because of running is unfounded. He is much better off now than before the ablation in that regard. It would be wise as itdood wrote that he take it easy at first and modify his regiment to a level that doesn't cause cardiomyopathy. Best wishes for his recovery.