The "Tasty Cheddar" is in October and follows the Avon under the clifton Suspension Bridge and takes in a bit of seaside at Clevedon and returns to Bristol via the Cheddar Gorge and Dundry. If you are already up to 40 milers, then I would say you have time to step it up and get through the Tasty Cheddar by the Autumn. Cheddar is at about the 40 mile mark in a 65 mile (ish) ride, and is where the relentless hills really start.
A great route, but Garmin is almost an essential (the printed route sheet, when you get near Chew Valley Lake is full of instructions like "straight on at crossroads, unsignposted, followed by right at crossroads, unsignposted and then left at crossroads, unsignposted" etc!).