4 miles (each way). Down Hills Road past Addenbrooke's Hospital, then left onto Worts' Causeway towards Fulbourn
To get to the start of the Roman road you'll need to cycle up to the top of the hill, which also happens to be the highest point within the city boundary, at all of 68m above sea level - still, it will give you a nice easy start to the ride back.
Once you're there you won't see a car for miles, let alone smell one! ...well, until you get to the A11, that is. By Cambridge standards the path is slightly hilly, however due to its Roman origins it is also dead straight. Fortunately, there are trees along most of the path, so you can't actually see very far ahead.
Don't worry about a route - just go straight. All the way to the A11 and back is 5.5 miles return. Alternatively, turn into Wandlebury Country Park and enjoy exploring it.
The Roman Road continues on the other side of the A11 (there is a bridge - mind the dog at the farmhouse). Don't forget that you have to come back all the way again.
If you prefer a one way run, you can leave your bike at Addenbrooke's and get a bus towards Haverhill, running back along the Roman road. Horseheath is at the far end of the road (10.5 miles from Addenbrooke's) or you could start from Linton (9.5 miles, with a small hill to surmount before you reach the road.)
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.