There’s a route I used to do from Bangor that went out to Abergwyngregin, and then up into the hills onto the roman road and past the stone circles. It aimed, via some wide open moorland footpaths to Sychnant Pass and south along more of the same to the roman road. It was 30 odd miles but very tough riding for the distance.
To lengthen the route, you could head further south to Llyn Cowlyd res. and go over into the Ogwen Valley. A road link would take you in the direction of Snowdon, either via Pen-y-pass or down Nant Ffrancon and over from Bethesda and through the slate quarries.
The trouble with the N. Wales I found was it’s quite fragmented as there are steep valleys and bit rocky mountains that you can’t always ride over.