Swimming - Yeah. There are four strokes but the medium doesn’t change and, for a given stroke, you always do the same relatively safe movement. It might get iffy in a busy lane but not much past that. (I won’t mention the level of effort because it holds true for SBR alike.)
Road Cycling - Very repetitive movement as well - whether you “pedal in circles” or stomp. Again, it might be a bit more stressful if the surface your are riding on is rough and the occasional pot hole that you can’t avoid. But when you swerve or change direction, you do it by balancing your body on the bike, not changing your leg position or your muscle action.
Running - Unless you are on the track or a treadmill, you have to sidestep rocks or go up/down the sidewalk or whatever and unlike cycling it has direct repercussion on your muscles/joints (you can’t hit the brakes when you have to stop fast or swerve to avoid a pothole). When you turn, you don’t just lean like you would do on a bike, you have to fight the centrifugal force*, etc. with muscle action and there will be more strain on your joints and tendons. (Very obvious while running in the snow/on ice.) Also, your ankle is always working to keep you stable on uneven ground (not so when you are clipped in on your bike). I guess that’s what I mean by more chaotic movement.
I am struggling to explain why but the bottom line is that there is a lot more wear and tear while running. Elite swimmers and cyclists can log 30+ hour weeks and most of the training injuries are overuse injuries. (Crashes don’t count.). Elite long distance runners won’t go much above 120 miles and it doesn’t take them much more than 12-13 hours to cover that ground and there are a lot more opportunities for tearing muscles or joints or even stress fractures. Can I hypothesize that running is 2.5 more stressful than swimming or cycling from those numbers? I don’t know. But is illustrate the point, doesn’t it?
Now, going back to Ironman training. A balanced peak week for somebody with Kona ambitions would be something along the lines of swimming: 3-4 hours, cycling: 10-11 hours, and running: 5-6 hours. (In any case, this is in the range described by the OP.) This five-to-six-hour running regimen is more stressful, in my mind at least, than the swimming and cycling combined.
In any case, my readings, backed by real life experience, tell me that swimming and cycling don’t necessarily require recovery periods (week or whatever) while running might warrant it on a regular basis (whether planned in advance or ad hoc).
*Centrifugal force is not a real force… but you see what I mean.
