How to train for that depends a whole lot on yuor strengths and weaknesses.

I'm big. And this being in the clydes area I assume you are too. But distance was never a problem for me. I've done more organized doubles than organized centruries. The First Double I ever did was the Grand Tour Highland, the lowland seemed a bit wimpy. BUT there were times on that ride I wished I had opted for the lowland triple.

If it were me and I was training specifically and lived close I'd be inclined to do the major climb as training. If I got to the point wher I could do that climb after about 5 miles to warmup and then continue on to the next smaller climb and turn arround and go back ot where I started I'd be confident for the century. If I did that working on the flat between the climbs both ways I'd be too confident.

But that is me and climbing feet are a far bigger problem for me that mere miles.

Calculating the grade it came out to 6%, that is an interesting grade, not killer, but enough to wear on a recreational rider. If you can find a climb just a bit steeper at 7-8% even if shorter it could be excellent training.