There isn't such a thing as a "runners body". Yes, having a lower body fat percentage will make you run slightly faster (for the same training) but aside from that there isn't a common body structure that suits running, successful runners come in all shapes and sizes. Sure, because she is already slim she may be a bit ahead of someone who may want/need to lose weight to aid their running performance, but other than that she shows absolutely no signs of someone who would be able to find training for a marathon easy.
Most long distance running talents are actually personality traits (there are some physical natural abilities of course, but those alone are not enough). To train successfully for a marathon you need patience, consistency, determination and most importantly willingness to go through pain, discomfort and to push your personal boundaries.
Considering one of Zoe's core personality traits is "liking being comfy and cosy" I find it hard to believe she would be willing to go through up to 6 months of consistent training, whatever the weather. I very much admired her efforts at the couch-to-5k, but she only seemed to keep that up for a month while it was a novelty and then got bored. You need a lot more grit to get up to a full marathon distance.
Sorry for the rant - I'm a running coach and don't like people assuming people are good at running just because they are thin. That's actually quite an unhealthy mindset! I wasn't having a go at the person I quoted - just trying to inform you all