Dude, I ran 1653 5k last fall, and my easy runs are only 730-750 pace. Make sure you don't go so fast that you can't recover from easy days.
Few tips:
1) Do pace work. seriously, just do it. Your body will become economical and efficient at race pace when you do lots of pace work, so you may be less fit, but you're so efficient at the pace because you run it so much that you can beat people more fit than you. With a sub 18 goal, this would mean 85 400s and 250 800s. Good pace workouts are 16x400m, starting a new one every 2min30sesc and hammer(all out) #12 and #16 to simulate mid-race surges. You could do like 8x800m with 400m jog recovery also.
2) 8-10 strides 2-3 times a week after an easy run they day before a workout to work on form
3) Its okay to just go out and do 3x400m at the end of an easy day then jog 5 minutes after to get some extra pace work in
4) A good fartlek to do, that former HS phenom Lukas Verzbicas used to do is 2-3 sets of a fartlek that goes 1-2-3-2-1 minute pick ups with equal recovery as the pick up you just did (i.e. 1 min pick up then 1 min recovery jog). Do a 20 minute warm up and 10-20 minute cooldown(depending on if you feel lazy that day)
5) 2-a-day runs. Run twice a day Mon-Fri, then long run Saturday or Sunday with the other day of the weekend off. This may mean just doing a 30 minute run and an ab workout in the morning 3 days a week and your main run in the afternoon, and the other 2 morning runs do 30-40 minute run and strides after for form work.
6) Ab workouts at least 3 times a week. A strong core helps you maintain form.
7) tempo runs and long intervals. Most people do tempo runs too slow. Alberto Salazar says tempos should be 15-20 sec per mile slower than you could run in a race for the distance you're tempoing. So if you do an 8k tempo, and you have an 8k pr at 530 pace, then you'll tempo 8k at 545-550 pace. Do tempos about once every other week for 8-10k, maybe 6k the first couple times as you get used to do tempoing. And if you're really weak aerobically, instead of the 15-20 sec rule, go by 20-25 seconds slower per mile. Long intervals may look like 6x1600m with 400m recovery jogs at tempo pace or 5-10 sec faster per mile pace than tempo. And another classic long interval workout is 10x1k with 200m jog rest at 10k pace or 10x1k with 400m jog at 5k pace.