One of the signature runs in Sheffield — and a good flat option — is the Five Weirs Walk, a paved path along the River Don that passes five historic weirs (low dams). The path follows the river for 4.5 miles to the Meadowhall shopping centre, with nice water views and a couple of on-road sections (well way-marked).
At Meadowhall, a couple of options: retrace your steps, or take the Sheffield-Tinsley canal towpath for 3.5 miles back to Sheffield, to make an 8-mile loop. It’s a gravel-surfaced path, a bit narrow in sections.
Longer Option: At Meadowhall, a path continues along the River Don/Sheffield-Keadby canal for another 3 miles to Rotherham. A fun option is to run the 7.6 miles between Sheffield and Rotherham, and then take the train back — it’s only 10 minutes!