Good luck with the sub 20 run. You're right in my range for the 5k. Two years ago I ran 20:35 4 days after running 4 miles at 7:21 pace which wasnt flat out (avg HR of 156 and my max is 187, finished at 167). Last year I ran 19:44 but it was during training for a half marathon so I wasnt doing a lot of speedwork. I did do a time trial of almost 5 miles at 6:49 pace about 10 days before my 5k (avg HR 167, finished at 179).Hmmm, five years reading this site (or it's predecessor) nearly daily and my first post will be to the running forum. Makes sense, I can run better than I can analyze baseball. Anywho, on New Years Day I've signed up for the local 5K. Right over roads I regularly travel though not on my regular running route. Hit the gym treadmill for 3.15 with 10 sec negative splits ending with 5:50. I felt I could have either held that pace or dropped it further had I not been trying dilegently to cut back this week. So, would it be unreasonable to think in my first 5K I can break 20 minutes (6:26 pace I believe)?
I'll try the pace option on my Garmin to see if it can help me hold the required tempo. The alarms are set between 5:50 and 6:25. Should the treadmill work make me feel like I can hit 6:26, I guess I'll know soon enough...
Maybe those times will help you gauge your time a little. Careful with those paces, 5:50-6:25 is a HUGE difference in a 5k. Do you have any other faster workouts you've done to better assess what you can run? Maybe an interval workout? How about other workouts? You'll most likely be able to go faster in the race than in training because of other people and the adrenaline.