You can do guided busway from St Ives to CAmbridge North station, then head past that to the river, follow along the river on Fen Road and Water Street to your choice of river crossing, and follow the signs towards Parkers Piece across the commons and through the back roads. You can then get onto Hills Road, which has reasonable cycleways alongside, and then onto Cherry Hinton Road.
One other way would be to leave the guided busway at Girton, follow through Girton to Huntingdon Road which has decent cycleways, follow that into town and you shoudl be able to head straight through the shopping district to the top of Hills Road. This is perhaps a bit more direct.
I'd expect true Cambridge dwellers might be able to add more.