Sorry, I realise that wasn’t particularly useful.
Start in Kilnsey, head up mastilles lane, then take the right hand track at the end, seen here (I hope that works).
Thats a nice trail, bit of grassy climbing but some nice riding after that. Follow it all the way to Hawkswick and back on the road to Kilnsey.
May be a bit shorter than 20 miles though.
Other option is Barden Moor, not sure how far from you that is though, park up in Embsay, then head out up this road, to the top of the hill, turn right onto this bridleway down to Bolton Abbey for an ice cream, then along the road to go back up the hill on this green lane to the go over the top of the moor on this bridleway, there’s a nice decent at the end, then back on the road to Embsay.
Hope thats a bit more helpful! (the barden route is the better of the two, but I think its a bit further away from you)