Stop your sceintifical!
I don't have a ton to add but.....My experience is that you can get a lot of bang for your buck with the occasional really long run/ride/swim. The advice above is going to be dead sexy for most endurance athletes. However, don't underestimate how much of your ultimate limits are wound up in being mentally prepared to suffer for hours or days.
My first mountain bike race that was over 50k seemed brutal. Once I had done a week long stage race, 50k desert races seemed like a temporary inconvience that involved some sweating and light chaffing. Overdistance stuff really helped my brain get fitter for the task.