The towpath along would be a good one for a visitor, as it is pretty straightforward, and you are unlikely to get lost.
You could follow the river, West to East. Running on the South Side of the river, starting at Jesus Green, crossing to the North side at Stourbridge Common, Water street and continuing East until you run out of tow-path or you get too tired. Maybe an 8 mile run could be go out of that, and if you want more, any number of circuits of Midsummer common/Jesus Green at the end.
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Also, if you want to fit a quick run in on a treadmill, you could go to the gym at Kelsey Kerridge Sports Hall, Gonville Place(as you don't have to be a member). I have done this before on a weekend visit to Cambridge, to refresh me between getting off the train, and going out on the town!
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Also, for anyone who has moved to Cambridge in the Histon Road/Arbury area and is looking for a short 3M local run, off the roads, there is a field off the Histon Road. Head away from the city centre on the Histon Road (the busiest part of the run), past Tavistock road on your left, and there is a track in a farmer's field. You could then turn left on a track back into a housing estate which includes Carisbrooke, Warwick Road etc. Makes a nice break from the roads. Or instead of turning left, continue the track which will lead onto housing estates off the Huntingdon Road (not so nice).