if you're unhappy with your time, you should add more mileage first, that's more likely to help you. i don't know how long your long runs are, but you should run at least one 8+ miler a week, and one or two 6 mile tempo runs are a good idea in addition to whatever you're running. in terms of a speed workout to help you, i would suggest running mile intervals, like 3 @ 6:00 (which you should be able to do if you're under 20, or you will be soon). i would also recommend a bunch of 400m repeats because they'll build more raw strength, and it's also just more fun to run fast sometimes to mix it up. I would guess you would do these around 80-85 seconds for each, with maybe a 100m walk in between, although at some point you should try to jog the rest interval, but wait until you feel ready
good luck!