Boggiest conditions I've ever met was on the Fellsman route. Crossing Fleet moss I went in up to my armpits and thought I was going to have to swim out, but luckily someone had walking poles and pulled me out. Further on coming off Great Whernside going down to Capplestone gate was also memorable, and this was in the dark having been on the go for 16 hrs. Finished with either trench foot or mild frost bite. Feet hurt for 3 weeks afterwards.
Well, there's bog and there's mud.
Many of the above races IMHO can be muddy (such as Marl Pits XC), but the worst bog I've come across is on Martcrag Moor on the Langdale Horseshoe race. Every year there is someone who 'goes in' up to their waist and needs to be pulled out.
I'm surprised there aren't more photographers at that point just waiting for the bogs' next victim :-)
See the light in the night
Never really associated the Lakes with nasty boggy areas... yes Martcrag is probably the worst i can recall on a race.
A race that went over The Nab would put the cat amongst the pigeons! Or possibly High Tove/Ullscarf...