No matter how ambitious your end goal is, all workouts and near-term goal paces need to be based on CURRENT fitness. The 3:30 marathoner hoping to get under 3:15 and the 3:30 marathoner hoping to get under 3:00 should both be doing workouts at a similar pace! Your muscles, nerves, and cardiovascular system adapt based on how the stress applied relates to their CURRENT capacity, not their capacity in 6 months of 6 years.
Do workouts to improve fitness based on CURRENT race times. If you're positive that you're much faster than your last race, do a 5k time trial on the track and prove it. That's a pretty good workout anyway.
Baby steps. Currently running 18:00 in the 5k? Then I don't care if you want to break 16 or 15 or 14. Let's focus on getting into the 17s first. For a newer runner, hoping to get 2 VDOT points of PR in a race is reasonable but ambitious. For instance, that would be going from a 5k of 19:56 (VDOT 50) to 19:17 (VDOT 52). 10-15 seconds per mile is a LOT to drop.
Many people here seem to know how to predict a race time from their fitness, and go out at a good pace. But there are also a lot of people asking for race predictions from completely inappropriate workouts. I suspect this is because they feel like that was their best workout, which is natural. To be sure you get a reasonable idea of what pace to shoot for, schedule some prediction workouts. These make really good capstone workouts to sharpen your fitness, provide a good indicator of pace to shoot for, and most importantly (to me, anyway), give a huge confidence boost. After successfully completing your prediction workout, you'll be able to swagger up to the line KNOWING if you just stay relaxed, you can hit your pace.
To be aggressive and ambitious, do a race-sim workout at GOAL pace, assuming goal pace is 5-15 seconds faster than current pace. For instance, If you currently run 6:00 pace for a 5k, but think you're ready to do better, try 3x1mi at 5:45-5:55 pace, with 45-60 sec rest. Ideally, do them as cut-downs, like 5:55, 5:50, 5:45. If you can do a workout like that in the middle of a normal training week, maybe 8-12 days before your next race, you're ready to go out at that pace and probably stick it.
Say you try that workout and run 5:55, 5:52, and then struggle through a 6:01. Well, you're not ready for 5:50, but 5:55 might be a good pace.
Say you manage 5:55, 5:50, 5:40, and the last one is hard, but you feel strong. I'd say 5:45-5:50 is yours for the taking.
Here are sample race predictor workouts for common distances. All are done at goal pace, unless specified, and should be gentle cut-downs (start a few seconds above goal pace, hopefully finish several seconds below). There are many variations on these, but these have become staples for me. All were gleaned from LetsRun posters or other sources - none are my own invention. I'm a student regurgitating what I've learned and found success applying to my own running, not a coach.
1500/1600/mile - I've never trained for the mile. Anyone have a good one? 400s and 800s of some sort seem popular.
5k - 3x1mi with 45s rest, or 5x1000 with 30s rest.
10k - 5x1mi, hammering the 4th - but be sure to leave enough to hit pace on 5. Then 800m all-out. 45-60s rest.
20k/HM - 4x2mi with 60-90s rest.
Marathon for ~3hr runner: 10mi easy, then 10mi at goal pace. Won't be easy, but you shouldn't need to dig until the last couple miles. I suggest making miles 8-12 a smooth acceleration, not jumping from E to M.
-and/or, at least a week part-
5-6x(1mi MP+10sec, 1mi MP-10sec) (if you go into debt on the -10 miles, MP goal pace is probably too close to threshold)