I'm a 22 year old male and have been running since I was 18 and for the last 2 years have been trying to get under the 5:00 barrier for 1600m (current pr of 5:04 set in summer '12). I do around 50 miles a week of running and have the typical schedule of 1 track workout, 1 20-30min tempo, and a 10-12 mile long run each week. Everything else is easy jogging at 7:30ish pace. I take an off day once a week as well.
I've been trying roughly once a month to go sub 5 and every single attempt ends up being 5:04-5:07. What am I doing wrong? Am I simply not built for speed? The 4 laps that I run at my mile pace feel pretty much like an all out sprint and no matter how hard I try I can't seem to go any faster.
My other PR's, if they mean anything:
100 - 16.7
200 - 33.1
400 - 69.0
800 - 2:26
1600 - 5:04
3200 - 10:20
5k (road) - 16:45
8k (club xc): 27:16