There's something about doing a 5 hour workout. Whether it's all one discipline (run, bike, etc.), or a mix of cross training. I find that in the week following a 5 hour I'm stronger in any discipline. It's just something about going that long on your heart and lungs and muscles.
I remember reading how Mark Allen recommends not going over 6 hours (ironman training) as it has diminishing returns. But "The Grip" also talks about workouts that are so ridiculously hard that they make your race seem easy. Imagine the confidence that can give you come race day.
At this time of year when I still have some tri's to do, the 5 hour is mostly bike-run. Come August I'll be doing a mix of biking-hiking....maybe some long 5 hour run-hikes too.
The Norwegians used to refer to their research where the heart size begins to expand after 2 hours of training. Bigger heart = bigger stroke volume = higher VO2 max = faster race! So a 5 hour is way out there in performing this important function.
It's interesting to me that if I train for 1/2 ironmans (as I have the past 3 years) instead of the full, I only do a 4 hour bike-run. But for a full I'll make sure I do 5 hour bike-run. You wouldn't think it's significant, but for some reason that extra hour just has an exponential effect...hence "The Power of the 5 Hour"!