Happened to me near the end of a 10km maintenance run. I was 65 at the time. Diagnosed at the ER as paroxysmal AF, which is what you have. Mine wasn't caused by running; it was finally shown to be from heretofore undiagnosed 'severe' obstructive sleep apnea. I endured the occasional AF until it began to increase in frequency. I saw an electrophysiologist (EP) who agreed to try a catheter ablation. Took two attempt seven months apart, but he nailed it the second go. I'm now 30 months free of AF.