Don’t beat yourself up! Running is definitely more than just “cardio”. So I say it’s more about your lack of training. And running outside is a different beast than indoors. The good thing is you can get back to training and running well again! It may take a little longer to recover or get to a nice base, but it’s doable. Adopt the run/ walk method first to get the legs acclimated with the distance. Time on feet!
This! So many crazy fast 50yr olds! Gives me hope.