Marathon-esque training is pretty good. The difference is really in being able to slow down so you are well within your aerobic zone, below your threshold pace/effort. Running slow enough so you can breathe only through your nose is a good measure. From there, it's about eating and hydration, etc. Ultras are often described as a "an eating competition with some running thrown in". The joints of course is a big deal too, some can do it easier than others.

Also, most people walk a lot, you don't have to run, you just keep making forward progress.

source: I've done a few Ultras and have used a running coach.

Alexandr had been running 185mi/wk average to train for this over the past 3 months. Lots of marathon and 2x marathon runs on back-to-back days. That said, there's an enormous chasm between someone like this training for a 24hr world record and your or I just training to finish a 24hr race on our feet.