My partner and a friend both did this event last year. They are both walkers at heart, but do a little bit of running.
The event is extremely well organised and supported with loads and loads of rest stops with loads and loads of food provided (for free) and ample supplies of water. There is honestly no need to carry any food/fuel unless you want something specific. Night - much colder than you expect, even in May, have lots of extra layers and the best head torch you can manage. Forget about running - you can't. Last year there was a lot of rain and the paths were ankle deep mud - loads of stiles - hundreds of people churning through mud and trying to get over stiles - yuck. do whatever it takes to look after your feet - quality shoes/boots + socks absolutely essential, and walking poles. And don't wait for anyone, or let anyone wait for you, stick to your own pace.
Last year there was was a training plan on the website which they both followed but the more time on your feet the better. I believe there are some organised training walks too, including night time walks. They both said it was horrendous, never again, etc etc and both are now planning to do London-Cambridge this summer
RunThrough did fab Womble and tennis medals for their wimbledon common half marathon series last year so don't think they would appreciate you doing same. Although no reason why not I suppose just doesn't seem right though.
A female cut tech shirt would be bloody marvellous
Are you saying he is overweight? At age 7 this is a much more serious problem then his running style. Be honest with yourself - a healthy diet? Really? Take control of his diet now while you still can and he will learn good eating habits for life.