Practice makes perfect (fun)
Of course, the kind of "practicing" I do is what athletic trainers call base building — operating below lactate threshold as much as possible, keeping the heart rate moderate, and maintaining a steady pace throughout long hours in the saddle — i.e., not slowing down later on in the ride. It works for me to practice this, because it's exactly what I'd do every day of a multiday endurance effort . When you have little time to recover for the following day, you can't pump a bunch of lactic acid into your muscles and rev up the heart. Everything you do has to be sustainable if you can manage it (Sometimes, especially in Alps foot races, even the slowest you can possibly move is still unsustainable.) But it's interesting, because even though I'm doing nothing for my high end, the threshold of my moderate zone is increasing. I can go somewhat faster with the same effort; but, more importantly, I can go longer.
Route info here.