The cycling training is fairly easy on the body and recovery is easier. Swimming is even easier to recover from. The majority of their training is very easy, with a couple harder efforts per week in each discipline. I will never do a 8h ironman, but have a 9:07 pb and this is what a typical week looks like for me. The best pros are prob doing double this.
Monday : AM swim 60 min easy + 5 miles transport run
Pm 120 min easy bike
Tuesday : AM swim 60 min easy + 5 mile run
PM track workout 60 min
Wednesday : AM swim 60 min hard + 5 mile run
PM 90 min bike + 30 min brick
Thursday : AM run 90 min easy
Pm Bike 150 min threshold work
Friday: AM swim 60 min easy
PM : 120 min run easy
Saturday: AM swim 90 min
PM 240 min bike
Sunday : off with some plyo/stretching