The amazing thing you find time and again is that triathlons are not that difficult to train for. Mostly because the kind of cardio you do for them tends to be the kinds of cardio anyone who wants to stay in shape will do. Of course, this ends with anyone who likes to run a couple miles a day thinking, "I should do this." And then they get eaten by sharks, the end.

"Best story ever." –all sharks (Source)

Seriously, though. The stress of this sport also comes from your body against the road. If you're doing it just to do it, there's the standard stresses of dehydration, hunger, and a loss of bodily control you get in any distance endurance sport.

Did you not know that? Oh yeah, if you do this kind of thing, you will urinate on yourself, and maybe your fellow athletes (source). Write that down.

If you're doing this competitively, then you run into fretting about your time. You're going to push yourself faster, because trust us, there will always be someone faster.