They start at Parker’s Piece, which I expect to be heaving because it is the only place with a big screen. They’ll head up Regent Street, St Andrew’s Street, Sidney Street, turn left at the Round Church back down Trinity Street and King’s Parade and then onto Trumpington Street and Trumpington Road. They will only start racing proper along Trumpington Road. The city centre section is all pretty tight, the width of the pavements is mostly 2-3m, narrower in places. I would think along the north of Trumpington Road by the Botanic Gardens would give a good view, since you can stand on the railings whilst still being walkable from town / the station, but I expect a significant number of other people will be thinking the same! Otherwise you need to get there very early or be high up.
Note that the railway station will be the southern hub for local buses, the southern park and ride facilities and all coaches for the whole of Monday. Expect it to be a bit mad!