I think I have the background to give a good run down. I have been a long distance runner now in my 43 year. I am 59 started running at 17 to stay in shape for baseball as a pitcher in college. I got serious about running in that I then started racing and doing marathons. I have 12 marathons done my last though was 1997 I pretty much gave up racing as I was in seminary to be a deacon. I have always been a cyclist but only off and on as running was primary. Then as miles pilled up I started cycling much more in 2008 and backed off running as much.. My lifetime running miles at the moment are around 85,000.
I never raced on a bike too dangerous but once I got the cycling bug it was great. The bad thing was I have developed runner's dystonia that allows me to only run/walk although sometimes I do ok. This is not an injury but a neuro issue, my brain has sort forgot how run correct. It is not progressive but comes and goes.. That said cycling has been my savior for getting in a workout. I always though running and cycling were just about the same although cycling easier as such. What I have learned as I cycle much more ( had 9600 in 2020), is they are very different.
Running a hard race all out will beat you up and you cannot recover like cycling. This gets even much more apparent as we get older. A marathon will beat you up like nothing ( except crashing) will on a bike. After all of my marathons I was really sore in the quads hard to go walk down steps and even get up from a sitting position. If you run a marathon all out are done for anything much other than easy running for at least a month or more. A hard 10 mile race will not beat you up that bad but it will stop you in your tracks for awhile with anything hard.
Cycling simply does not do that much at all. One can be sore a bit if you climb or maybe at first getting acclimated but just not the pounding. I have done some very hard solo century's at near 5 hours and they do take a toll but you can keep that up just backing off to let reserves build back up. A few days at most if well trained. So in that sense I will say the running is harder. But the thing I have noticed about cycling many miles is that it will build up fatigue level in body different that running but sometimes as challenging with respect to energy level. Not in physical beating up of body but it can wear the system because of the very fact you can just keep riding pretty hard days in a row. Finally though it will hit home even those in the Tour De France realize that quickly. My fatigue level in cycling is just different and it can effect how we simply do in getting around. I noticed this year when I was doing some 900-1000 mile months that I need plenty of rest and down time. More than say a 40-45 mile week of running would do easy. You don't get beat but it can deplete and we need to watch out for this.
For me the bottom line these days is yes, I like cycling, do that primarily and run as I can. Cycling is more interesting in the equipment and all needed to ride so that can be fun. The biggest problem with cycling is that it can be very dangerous in crashing and traffic we know the stories. The other problem with cycling is that it does take longer to get the same effect as running. The moral for me is that they are really quite different aerobic training and general hobbies. They can do the same things in benefit of heart and lungs but very different body effects. There is nothing like going out and running a really nice 5 mile run hard and being done in 40-65 minutes. However it does not give the thrill and the views of riding along and seeing the beauty of the land. The thrill of a fast descend will never happen running.
For me cycling has been a blessing. I love running but generally no cure for dystonia and maybe much better off as we get older. I am retired from my civil job so cycling works well and I lost my bride 2 years ago after she battle beast cancer for 10 years. Cycling keeps me focused and allows time to think. Remember the deacon comes up with homily material on rides all the time. I say to cycle, run, walk, and whatever keeps you active. Then of course it helps if you like good beer, donuts, ice cream, and cookies.