Having had to give up running a few years ago, I wondered if I could do a V60 Joss Naylor Lakeland Challenge in the permitted 18 hours, walking all the way. It turned out I could keep to the schedule on all of the legs (I'm a pretty fast walker).

However, the Joss is about 77km / 5200m whereas the BG is about 105km / 8200m. Extrapolating my times for the BG would've made it about 27 hours. I'd say you'd have to walk VERY fast to do a sub-24 BG.