Run round them rather than through them, don't split calves from their mothers and Bob's your uncle. If they trot after you, don't worry but if they start catching up just turn and face them, waving your arms and shouting 'yarghhh'. (Thats 'yarghhh', not 'yargh' or 'yarghhhhh' mind

). It tends to work for me anyway.
Mind you when I'm running with Harry my border collie, some cows really, really do not like him and none of the above applies. In fact all sorts of fun and games can kick off.... but we always manage to survive.