no improvement?

last year's platte river half - 3:04
this year's platte river half - 3:01

three minutes is not much improvement.  what am i doing wrong?  i (incorrectly) assumed that i would naturally get faster as i put in more miles.  i hit a little over 800 miles during last year, so apparently mile-time is not helping.

i don't do speed work.  i don't really know what doing speed work means.  but if i am serious about wanting to improve, it looks like i need to figure it out.