And, it has been this part of the trip that I've been the most concerned about: after all, if a hill gets too steep or weather gets bad, worst-case scenario is that I can start walking the bike. However, it was really an open question for me exactly how I'd carry enough water to make it through two days in that barren desert, at least without tricking out the bike with front panniers (which I was resisting given that this is the only point in the entire trip where they'd be needed, and given that I'd need to get them installed much earlier in the trip).
But, here's my current plan: when I arrive in the previous town of Alpine TX, I rent a car and drop off collapsible (but sealed) water containers at various points along the Marathon-Comstock route. I'll drop off enough to where if one gets missing or destroyed, another will be coming up soon enough. As I go, I can empty the ones I don't need, and take the empty container with me. If I want to get fancy, I could even stash the sleeping bag and sleeping pad at some hidden location midway through the 144 mile stretch.
For my later reference, here is a link to the Alpine TX car rental business.