Running & Weight Training?

Which do you think is more appropriate for a runner who trains mainly for the 5K: Endurance reps, so lifiting sets of 20 or more, or strength reps, such as sets of 10? Its hard to find a happy medium because I know in cross country running an 8K it is better to do reps, and sprinters do strength lifting, but I don't know which is more beneficial for middle distance track?