Speed is a good tool to have in your toolchest.
So is the ability to have short quick stops, fix a flat quickly, sleep anywhere, navigate, clear your mind, and self assess your mental state. The more aware you are of your body, the more you're able to push it close to its limits without getting into trouble.
There comes a point when you have to stop thinking about it, obsessing over the numbers, and just do it. Honestly, you don't know how your body or (more importantly IMO) your mind is going to handle the realities of long distance. Don't overthink it.