Hi. Just catching up on the thread today and thought that maybe I could help with this.
Yes, walking the Marathon is completely possible, but training does need to be done and taken seriously. I've walked the last 2 and trained using cumulative fatigue training plans. This year I trained 6 days a week (my preference) and that included easy A paces, easy B paces, LR paces, M paces, Hills and speed work. Just getting out and walking some miles during the week won't be enough. My previous plan had me going 5 days a week and got me through well enough, but this year I felt a whole lot better both during and after the race. Some of this I can attribute to lessons learned from the first time, but I feel like this years plan prepared me better. My longest training walk was 11 miles, but I put in 30 miles total that week. I was originally concerned about not doing any longer mileage, but if you're doing more during the rest of the week it helps balance everything out.
With all that said, I was very happy with my sub-7 hour time this year. Go Team Not So Fast! I averaged a 15'29" pace that included one bathroom break and some characters. Looking back at the times, once I got warmed up I stayed consistent throughout the race and still felt good at the end. I could've stopped for more characters, but my goals changed during the M when I saw I could go sub-7 so I didn't stop as much as I could have.
@sandam1 is correct though about the difference between the half and the full. It's a bigger jump up to the full than going from a 10K to a half. Many more things to think about and plan for that can affect how things will go. It really gets down to whether your Mom wants to do all the training and make the effort to do a Marathon. Hope this helps.