I haven't done a marathon but I have ran 17 miles at about a 9 min pace. Once you can run 10 miles or 12 or 15, it just feels normal. You need to build your body up physically to train for the distance (bones, muscles, tendons, etc) but 17 miles felt as hard as 10. I get bored after 45 minutes of running, so it was key to keep music or podcasts on for me as well as water.

If you're looking for a specific race time it's a different story. Getting your endurance up to a marathon is probably the easiest part because it just comes with training. Getting your body physically ready so you aren't injured is much more difficult. Proper nutrition, sleep, training, gear, etc can all help.

I stopped at 17 miles because I developed stage 2 osteoarthritis in both knees and it ended my running career after 5 years, 508+ runs, and 2300+ miles.