Fill your boots on the Derwent Walk from Swalwell up to Blackhill. Unnoticeable slight gradient along its 13 miles. You could get the bus back.
Good 7 mile there and back around the southern edge of the Derwent reservoir from Powhill car park, all on good manufactured paths.
The old railway line up on the moors by Waskerley is a level walk, about 4 miles up to Parkhead café and then back to the car park