Sorry, but at the risk of being controversial and hurting some folks feelings, run/walk really doesn’t count, it just makes you look like an under prepared muppet.
I trained hard for the Snowdonia Marathon in 88 and still didn’t manage to run all the way, taking 3.29 to finish. In 97 (aged 36) and not running much at all, a friend asked me if I wanted to take a mates place in the London Marathon two days later, as he was dropping out with an injury.
Always up for a challenge, I took it on, but decided to just ‘get round’. I ran 9min miles to the 20m mark and from then on, suffered/walked/jogged to the finish as Dalmations et al overtook me. I finished in 4.14.
So as others have said, its doable, but take it very easy….you can always finish fast!