Endurance you say! Yes, revising is an endurance sport. Like long-distance running or lake swimming (as in swimming across a lake - a wonderful sport beyond my ken at this point). Once begun, one must endure, even if you feel your energy lagging. It helps massively to have a deadline, even if self-imposed and without any threat to health or property. I have one, this time around, as I've engaged an editor to help me get this ms in ship-shape for selling. I will have had roughly seventy days to get this from first draft and details (beat sheets, character analysis, and back story) to 2nd draft. I have about forty days to go. Eep. At that point I will send it off to the editor and she will go through it with a comprehensive edit. Then back to me for changes, and I have a few worthy types who will give it a copy edit. Then, hopefully, out! During these few months of the revision, I have to keep plugging, day by day, making sure I'm not spinning my wheels in the mud of weird plot points, and moving towards that finish line.
I did the Dublin marathon when I was fifty - walking, but it is just as long. the main struggle is with your mind. My body can walk that long, but my mind tries to convince me it can't. Working on a novel altogether is an endurance sport. I can write a poem in a couple of days - maybe revise it a few times and done. Even a full-length play can be done fairly quickly. A short-story takes awhile, but truly - no comparison. With this novel I got 80 thousand words done in two months. But it has been simply years of frigging around since then. Now I can see the finish line and I must keep my pace steady - work in my splits - I know near the end I'll have more energy, not less, so might get more done. I've figured it out intuitively so far - that I will slowly build up to working longer each day, and give myself time to fall, get stuck on a plot point, etc... by building in more time than I need. Endurance is a mental game - not panicking and working too hard and burning out, but developing a pace that suits my temperament, age and desire!
Here is a photo of the Dublin 2002 race. I'm in there somewhere. I'm wearing green if that helps!