A marathon...that's 26 miles. Could you do it with now? With practice? In a dire emergency? It sounds crazy and I wouldn't expect you to run 26 miles but it is necessary, in many survival situations, to be able to walk yourself away from a disaster, a war zone, or other unsavory situation. Besides being a necessary survival skill, walking is also an easy way to exercise, and best of all, it can result in lowered blood pressure, lowered blood sugar levels, and lower cholesterol.
Think of all of the people who suffered in the Louisiana Superdome for more than a week after Hurricane Katrina. Yes, people said they couldn't flee the city because they didn't have a car or couldn't afford gas, but aside from the ill, elderly, and those with many small children, I wonder why they didn't use the two feet God gave them to get to higher ground. I know, it's easy to say "why didn't you...?", I wasn't there and have no idea what other factors influenced these people's decisions, but the ability to take yourself from one location to another (no matter the distance) under your own power is necessary. If nothing else, walking a few miles or more every day will result in better health, but in a disaster it could save your life.
Here's some walking resources: