I benefit from 6 or so miles a day dog walking, and leg day scheduled 3 days a week in my home gym. Mucho respect to the top flight triathletes who put their bodies though all sorts of contradictory exercises, but who still manage a high at both. Not easy.
I come from a club running background (after playing football into my 30's)` - reaching a decent level without troubling the fast guys - back in the 80's when running was the thing to do. After 5 ( and a bit) marathons and regularly training >50 miles per week, I decided that too much pavement pounding was not doing my knees any good, I decided to take up triathlon, which got me my first introduction to cycling (but did not turn me into a cyclist). The next winter road race season, in my 40th year, I turned in a whole pile of pb's while only running 20 to 25 miles per week as part of the triathlon training. What I found was that the cross -training was a lot easier on the body, but the fitness level that was achieved was way above anything I'd managed in my younger days. After a marathon I'd be hobbling for a week. After a triathlon I would be out running the next day. Not that I was that good - usually in the top third of the field, having lost out in the swim and then overtaken lots of people on the run.
After all that, the point I'm trying to make is that the elite triathletes are probably not training any harder than elite athletes in any other discipline, and the cross-training may make them less liable to injury because they are less liable to be pushing any single discipline to excess. Mentally too, triathlon training should be a bit easier than training for a single event - though the standards achieved by the elite are incredible.
This doesn't mean to say it's easy, though. Anybody can run, but these guys are running at top class club runner level or better, cycling demands a level of skill and understanding, as well as fitness, and swimming is certainly a specialised skill demanding good technique and good fitness.