Midsummer Common - 6 miles return, or Pike and Eel Pub (Chesterton) - 3 miles less (return)
Early mornings in Michaelmas Term: the havoc on the Cam has to be seen to be believed - definitely a must.
Except for Chesterton, there are no cars but it gets kind of monotonous once you are on the towpath.
Follow the Cam eastwards, initially on the North side. On Stourbridge Common, you need to cross the river and pass through bits of Chesterton to the Pike and Eel Pub. From there, keep going along the towpath. Go as far as Baits Bite Lock on the other side of the A14.
Optionally, do not cross the river at Chesterton and run over Ditton Meadows to Fen Ditton (3.5 miles return). There's also a little path to the lock - wahey, a round trip!
The towpath continues for miles; the next bridge is at Clayhithe (altogether 10 miles return) right next to Waterbeach. There, you may cross over and run back along the road and through the village of Horningsea before turning right on a footpath that takes you back to Baits Bite Lock (add an extra 0.5 mile for this option). This allows for the remarkable possibility of a figure of eight loop!
There is also a public footpath from Clayhithe to Horningsea, some hundred yards east of the road. Nevertheless, you will still have to enjoy the tarred road through Horningsea.
Alternatively, you may take the train to Waterbeach and run back from there (5.5 miles). After Waterbeach, the train calls at Ely - that would be about 17 miles.
The author cannot be held responsible for any kind of injuries, trespassing violations, or anything else occurring in connection with following the stated routes. Anyone may do so only on their own risk.