I became symptomatic for a heart condition called Mahaim Fiber Tachycardia in 2010, not long after completing the Great North Run.
The consultant believed it was something I had from birth, but that the high intensity training and half marathon had changed the shape of my heart enough that it became symptomatic. His opinion, not mine, before anyone says anything!
If it was the reason, then I’m glad I did that training, as it meant I could be treated and remain symptom free ever since, otherwise who knows how it could have manifested in later life.
I have cycled lots since with no issues