Two different questions!
It’s quite easy if you go slowly, it just takes time.
It’s quite hard if you go fast.
Also depends massively on the conditions and route too, your route is pretty flat so should be reasonably quick depending on if you stop or not and if you have any navigation issues or towns/lights to negotiate.
I do similar mileage to you (I may not be as quick) and my 100’s vary a lot, a hilly Dartmoor 100 with 10,000ft+ of climbing will see my average <14mph, but a flat 100 with a group of people could be 18mph+, and I know people in my local club who have done Ride London at an average in excess of 21mph, who would be puffing on a 17mph 50miler so I don’t think RL is much indication!
If you don’t regulary ride longer rides you might find pacing or fatigue an issue, as you may have trained yourself specifically for going hard for a couple of hours or so, but struggle with longer rides. Then again you might not!
For a first attempt at flatish 110miles Give yourself a 7.5hr target (~15mph average) and see what happens 🙂
EDIT!
My 40 miles has about 2500ft of climbing and takes between 1:35 and 1:50 hours…
So you’re quite quick then (>25mph average! still >20mph on the longer time), If so then you should be able to do 6 hrs or less.