If your GF doesnt ride, id suggest that anything around Grinton or Castle Bolton will just about kill her. I live in Richmond…there isnt much easy stuff due to the hills. Northallerton is flat as a fart, and its all farming country, so the BW’s are basically muddy fields, avoid at all costs.
Id suggest your best bet would be a road ride from Richmond to Reeth, enjoy the gorgeous scenery of Swaledale, 11 miles and slightly uphill mst of the way, but the scenery makes it easier, then stop for cakes at one of the tearooms…then ride back again.
An alternative would be to turn right afetr about 4 miles towards Marske, turn left at 093032, for a short steep climb, R at 084016, then R again down by Telfit Farm, then enjoy the downhill through Clints wood, back to Marske, then the B road past Beacon Plantation back to Richmond. ALternatively turn R at 114019 for the BW back to Richmond – though this can be hard to navigate thanks to the farmers up here who hav out stiles all over, even though its a BW. Grrrrrrr.
Have fun.