Start at The Mill (End of Mill Lane) - 3 miles return
The bit alongside the brook is very enjoyable and feels like a real forest at intervals. Unfortunately, that feeling doesn't last for long.
Go down Coe Fen on either side of the river. At the Leys School (southeast of Coe Fen), follow the path eastwards and head towards the traffic lights which you then cross. The Vicar's Brook that runs alongside the Botanic Garden continues southwards (actually, it comes from there). Alongside it, there is a path that you follow all the way to the Ring Road (Long Road).
Do not cross the Ring Road but turn left onto a little track that will lead you underneath the road bridge. Continue southbound on the guided busway cycle path.
Instead of running the same way back, you may cross through Trumpington to Grantchester and come back through Grantchester Meadows (round trip about 6 miles).
On the guided busway cycle track, take the next left and cross the railway (not dismantled). Turn right and after a short stretch along the road, turn right again on to the DNA cycle path. After ~ 300m follow the path round to left and the small wood (this is Nine Wells, the spring of the brook - Cambridge's source of fresh water in the late 19th century. Check out the little obelisk inside). The obvious hill behind it is good for hill training (this is the place where the Hare and Hounds do their weekly hill sessions, so I believe).
A small loop can be added by following the track up the hill, past the farm on the other side to a road. Turn left up the road then left again onto a footpath that leads you back towards the railway line (altogether about 8 miles return). It is also possible to continue out towards Wandlebury, mostly on footpaths.
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.