You're right, from Arkengarthdale you'd need to do 406 km in 45 ascents from Great Punchard Gill, which is the bridge just north of Whaw. Stonesdale would see you do 268 km in 45 ascents from the River Swale; that's a pretty good distance for Everesting, not many people do significantly less than that. From Stainmore you'd do 311 km in 28 ascents from the River Belah, so a bit longer but not tragic.