I think the two things to think about
1)is the amount and type of climbing much rougher and steeper.
2)the lack of stops, There is always plenty of resting spots on the Fellsman with plenty of food but on the BG you have 4 of which depending on shedule might be a total of 40 minutes

The first 3 times I did the Fellsman were in the 19-24 hour time bracket as a walker. Last year I did 17.27 and felt really good at the end. I have no worries of the distance or being on my feet for 24 hours but the climbing....

It depends on what you are good at, but I would put it at least 50% harder if not twice as hard.

There are plenty of attempts going on in the next 6 weeks so why don't you try do a couple of legs, It will give you an idea of what it's like.

My first experience of the BG was helping Plumbum do his and I was wrecked after the first leg!

I take it you did the Fellsman this year centaur what time did you do?