Hi,
I'm not doing the velothon, but I do a fair bit of cycling (I've just returned from a 400 mile tour of Ireland including the Wicklow mountains) and I've found it a superb activity for someone with type 1.
My main advice would be not to overeat! I've found for a distance ride my best practice is to eat normally beforehand, take a little less short acting than normal, and let the exercise use up the sugar. This takes practice to get the numbers right of course, as with everything we do.
Also don't munch glucose before hills. Made that mistake at the beginning. The stress of climbing causes your muscles to produce cortisol, leading to glucose release from the liver, so again unless you've used up your reserves the sugar will be there. After particularly hard climbs I test to see how high I've gone. Strange but true.
And of course carry food, not just glucose but stuff like granola bars to pump you up and keep you going when the cycling seems harder than it really should be.
The route looks wonderful - wish I could join you!