If you just want to complete it, most marathons give you 6-7 hours, which is a little faster than walking pace. Anyone can walk 5 minutes and run 2 min all the way to the finish line.
If you want a respectable time, you need training.
1-3 miles every other day for a couple months.
Then 3-5 miles about 4-5x per week for another couple months and then start a basic marathon plan. It really does take several months to get the tendons and ligaments strong enough. And it’s really tough to restrain yourself because you have the cardiovascular endurance and the muscular endurance to go longer. It’s the musculoskeletal integrity that is the issue for new runners.
As far as your question is concerned, it you can do 13-15 mile long run, you can definitely do a sub 5 hour marathon. Anything more ambitious will require real marathon training.
