The Cambridge Hare and Hounds host their own half marathon/marathon event each Lent term. With its scenic route and cheap entry it is hugely popular with students. Limited to 700 places so make sure you sign up quickly.
Link to online entry can be found here: http://www.cuhh.org.uk/competition/boundaryrun/boundaryrun2014/
Entry cost is £10 for students this year (t-shirt and free food included).