When I was young and fit I played football and was told I had a 'good engine'. I started running at lunchtimes in my early 30's to keep fit and there was a good club at work. This led to racing with the club on road and cross-country, which eventually led to finishing 5 marathons, 2 of them in under 2h 50m. The year I turned 40 I started triathlon training and competed in a few - the longest being a bit longer then half-ironman. At that point my swimming was just about adequate and I'd very little cycling experience, but my general level of fitness saw me through. The year I turned 40, with a few months of triathlon training, I ran pb's in several races. Then I burst a hamstring playing 5's and my running went from 6-minute miles to 8-minute miles. I didn't cycle at all after that for a long time, but I kept up the running (more like jogging).
Years later, after redundancy, I got a job about 9 miles from home and started cycling to work. Cycling fitness improved rapidly and I loved it to the point of joining a club. Now, several years into retirement, I still love my cycling - though golf is my main sport - and I'm still doing a couple of thousand miles a year (none of these cissy kilometres for me).
It doesn't really matter what level of fitness you are at, there's no doubt in my mind that cycling is much easier on the body. Cycling and running are both hard on the inexperienced, but with cycling you can take it easy in a way that you just can't when you are running. After running a marathon, I'd be hobbling for a few days - even after weeks of 60 - 70 miles a week in training. After a triathlon or a big cycling event, I could be out training the next day, albeit in recovery mode.