Hi Hip Priest. I'm doing the 104 (this will be my fifth time doing that, and I did the 63 miler in 2008).
What are the hardest hills on the route?
Well, it's all relative to your experience I guess. Earlier I was looking back at some old posts/threads about the Ryals... first time I did them was hard, but I've done them 3 times this year so far, and no probs. However, last year, I stupidly did the cyclone with a hangover and had to walk a bit on the first Ryal.

Won't be doing that again!
Otherwise there's Ritton Bank, which is the first big hill, at about 21 miles or so. Short, but steepish. Saw a bloke break a chain halfway up last year, and smack his nuts on his bike

Again, I ride this hill regularly, so it's not a prob. for me.
Billsmoor, about 45 miles in is another fairly big one (and yet again, without boasting, it's a regular ride for me). There's a long slow biggy at about 60 miles, or just before. Then the Ryals.
But I guess the wind needs to play ball. Hopefully by being absent.