I do agree with many of the posters who say that increasing mileage is important . But in my 10 years of running in middle school, high school, and college there is one thing that I found out was true for me and that is you need to run fast to run fast. It sounds silly but I think that it is an important idea to keep in mind. If you want to run 6:40 pace in a race you need to run some mileage at 6:25 pace (just an example). So I wouldn't be afraid to drop the pace of SOME of your runs. During your long and easy runs the total run time should be your only concern, not pace.
One bread and butter workout that always helped me was 6-10x800m at 10-20 secs below race pace (per mile) with a 30 second rest. The important thing to remember in this workout is to hit the pace if you find yourself falling off take a little longer break than usual and try again or just stop. Because if you can't finish them properly then you've probably gotten all you can out of the workout. Also remember that this workout is all in the rest so it is critical that you keep it short. You can adjust the pace depending on how you are feeling.
I would do this workout every other week mixed in with a fartlek workout of some longer reps over the summer. This should help your endurance and the speed that you are going to inject over the season. This way the speed won't seem so foreign to you when you get to that point in the season. A good fartlek for beginning runners that I used to like was Telephone Poles. Of course if you don't have telephone poles where you live this could be a problem but the premise of this workout it that you run say 8 telephone pole spans hard then 3-4 easy and so on for 4-5 miles. You can adjust this based upon your fitness level. You can do this with minutes as well but I think that the telephone pole thing spruces the workout up a bit. The goal here is to run at a harder effort than your normal run pace but for longer than the 800m reps I suggested earlier.
Of course you are just beginning so remember what I have suggested is only true after you have been running for at least a little while. Don't go and get yourself hurt. Good luck. And remember what worked for me may not work for you its just a suggestion.