trouble with going down one of the white hill / puke hill varients is that you end up losing a load of height back to the river on tarmac down from the car park. Must be more singletrack from there down to somewhere on the river?
There is (assuming you mean the car park at the foot of Puke Hill). There's a tasty wee piece of singletrack through the trees on your left hand side (opposite the Scout camp).
Losing the height isn't a problem for me as I live in Bonaly, so that bit is usually the end of my run.
Alternatively, from the car park there is the track that leads off to Torduff, or the other one that leads round the back of the Scout camp and down towards the Polo Field, or continues on to the foot of the Dreghorn climb, or even on from there towards Hillend.