Head out B'thorpe Road. In Bish, take the left turn immediately past the sharp rh turn. Continue past the Ship Inn. At T past the Ship, turn L and head out through Acaster Malbis. At T turn L. This takes you onto the airfield. Follow round. At T, turn R for Appleton Roebuck.
In village, turn L. Follow road over the railway bridges, then turn left for Coulton. Just follow the road round to Copmanthorpe. In Cop aim to head back for Bish (R at T, R at next T, second R (signed) and follow your nose back).
It's pretty obvious on the ground.
gmap pedometer says 17 miles.
http://www.gmap-pedometer.com/?r=1361959
You can extend it by looping round the airfield at Appleton Roebuck - just turn left instead of right and keep riding wihout turning off and you'll end up back on the airfield for another lap.