the whole thing! It's not that long.
FWIW Kielder is not on the way ANYwhere. start at the bottom, climb all the way to the radar station & do the whole red descent & take in the black bit at the end 🙂 or if you are short on time drop the climb to the radar station & turn right where the radar descent crosses the access track.
so in summary do ALL of it – unless you are a jey who dislikes rocky drops, in which case go & do the red route at Newcastleton.
Newcastleton is more like a welsh red – smooth & flowy singletrack pretty cool. esp. for a XC type hardtail, but not "G-narly". my 11yr old loved it 😉
