I have run now for 41 years and still at but very slow these days. I think I have done 12 marathons but last one in 1998. I did manage to get to at 3:06 for a PR. I manage to keep running now about 30-40 miles a week but very slow 10 to 11 minute miles. I seem to not be able to stride correctly like I am off balance it has been going on for 10 years. However on the bike I am fine and frankly I do think the cardio for running can allow much better training than the bike alone. The problem runners get into when they go to cycling is they need to work on riding skills and technique. I find if I am not riding a lot because I it is winter and I am running I need some time to adjust. Getting up out of the saddle to pedal can be awkward at first after a layoff.
Finally on another topic. I run on the treadmill in winter a lot but now I cannot seem to get up an run on the treadmill. I fell a few times tripping and now I am sort of freaked out and cannot manage to break into a running form on the treadmill. Maybe after 41 years my legs are just done...…….but they don't hurt just bad balance.