Before the ramble it would be wise to drive up to Great Chesters farm and ask Mr or Mrs Woodman (the farmers) for permission to go through their farmyard.
From Walltown Quarry car park return to the road, turn left and shortly left again. After some distance the road becomes a track. Eventually tarmac starts again. After a while take the rough track uphill to Great Chesters farmyard (ensuring any gates are closed) and then on to Cawfield Quarry for lunch (disabled toilet). If you wish, you can continue on the road past the entrance to Cawfields Quarry car park towards Cawfields farm then turn right uphill and through a gate to gain access to Milecastle 42.
Return is the same way.
If you have the time and energy when back at Walltown Quarry there is a network of paths to explore; ask at the kiosk by the toilets for a map.