Follow the Derby canal from the centre of the city to the Trent and Mersey Canal at Swarkestone. The flat path runs for about 6 miles and is suitable for cyclists or walkers.
The route starts in the city centre and follows the River Derwent past Pride Park football stadium and The Sanctuary Bird and Wildlife Reserve. You then come to Alvaston Park where you turn south away from the river, following the path through the suburbs of Allenton, Boulton and Shelton Lock. At Swarkestone Lock the canal ends, meeting with the Trent and Mersey Canal. You can extend your exercise by heading east or west along the canal.
You could also continue south along the Cloud Trail to Worthington.
Another good option is to continue following the river east at Alvaston Park and head to Elvaston Castle Country Park. It's only a couple of miles away and has a lovely lake and nice woodland trails to try. Just to the north of the canal there's nice riverside walks through the popular Darley Abbey Park.