Ya i agree with bruncle. I run the 400 and 800 and i only lift to improve my running form. Just start training and doing a lot of speed workouts. And i dont mean short sprints, i mean real good hard workouts. You won't really be able to tell how much your times have improved until you race though.

I don't know if it will help you but I'm gonna add on to what kirby said:

High reps with lower weight builds endurance/ tone your muscles; low reps with high weight will build strength/mass. Another thing you might try, once again I'm not sure it will help is to try speed lifting. Some people say speed lifting doesn't do anything but it improves how fast you can get the weight up which can improve how much weight you can lift which might make you faster.

1 set empty bar/ low weight
3 sets of 5,4,3 low weight, you're going for speed not strength

Lift the weight as fast as you can (while controlling your motion) then lower it at a normal speed, I think the general rule of thumb is 4 seconds.

I'm on a 5 day lifting plan and I split this workout between Friday and Saturday after a rest day so just do it one day a week if you try it.

Like I said, I'm not sure if it well help and I'm not sure if it will hurt. I'm more of a lifter than a runner; but I'm trying to improve on my running. Good luck