I run my best 5km when I'm doing more miles and running more frequently (but not necersarrily doing more speedwork).
Economy and the ammount of upper end aeorobic training you do has more impact (IMO).
I would agree with Morahan is saying about it taking a few weeks of targeted training to get in decent 5km shape (from a distance background). From a period of big miles, If I start hitting a few time trials / 5km sessions then my time will really come down.
I think I ranged from 16:40 to 15:43 5km in the space of a few months, just by running more miles and adding abit of focus for the last few weeks towards the end.
One thing I do find though is that 5km is a difficult race to run well.
Gobi - Fast enough to run 2:35 ? I'd be happy sub 2:40 but i'd like to run under 6m/m. I ran 1:14 for a Hilly / undulating HM in May. Mcmillian makes that a 2:36, given that Shakespeare is only a bit hilly I'd like to crack 2:35, never raced 26.2 before though so its all a learning curve.
I'm racing nearly every Sunday now until Mid April, hopefully this geared with a midweek LSR and 90 peak mileage will hopefully work well.