FWIW, after the NY Marathon, LA said he had never felt so totally exhausted at any time during his bike racing career. The fact that any fit person can cycle 100 miles a day or more tells you something. Sure, there is cycling and racing, but the fact that riders can race 100 miles a day tells you something. You can't run a marathon every day, not at speeds remotely approaching what you can run on a one day only basis. This is simply because running taxes all the major muscles of the body, not just some of them. The recovery needed from one marathon is far greater than the recovery from one 100+ mile bike race. It would probably be a little different if bike racing didn't involve drafting. It would be harder to do 100 mile TTs every day. But I'm sure racers could do it at speeds not way, way off from what they do it on a one shot basis. Look at the fact that at the end of the TDF, contenders rider ITTs at very high speeds.
In the end, of course, if one sport makes fewer demands on some skills than another, then the cream of the crop spend all their time focussing on just one or a very few skills, to a degree that would be impossible in a sport demanding many skills. So one can always make the argument that any one sport is just as demanding as another. Since bike racing doesn't require a lot of training for agility or hand-eye coordination, racers put proportionally more of their time into endurance training. It's not like they're having it easy, in their own way they're making just as much efforts as athletes in any other sport. But I have the greatest respect for sports that require the widest range of skills, and I just don't see that in bike racing.