That’s why there are programs like Couch to 5K which slowly work up to a nonstop 30 minute run. It doesn’t even take that much determination and it’s practically designed for obese people. I was able to complete it weeks ahead of schedule at a BMI of 30 and the 4-5/wk runs it enabled helped me lose 35 pounds in 2024 - the vast majority of the weightloss happening during the weeks that I ran.

Nitpick: a marathon is 42 kilometers (by definition). Most people would struggle to even walk that.

There's no such thing as a 5km marathon, that's a contradicition in terms. However, if one were running 150 minutes a week on a regular basis then a (42km) marathon would likely not be too intimidating a prospect. Suffice to say, most are not getting in their doctor recommended level of exercise.