You'll be fine. Chevy Chase (though I haven't ridden it) is considerably hillier than Gilpin's Gallop, but there's a couple of months between the two, so that's plenty of time to train yourself up.
I would recommend an intermediate distance, so that you build up the miles. A ride of 140 or 160 kilometre in the middle of April would be ideal. 162 kilometres is near-as-dammit 100 miles, so that would make a satisfying target, and a useful bridge to the 200 km.
I'm only in Darlington, so not that far away. It's good to have another north east member.