More miles. Speed work is good, but most experts caution against it until a person has been logging 30 mile weeks for 6 months or more. I would not recommend anything more structured than a few fartlek intervals built into a workout or two every week.

How I know more miles is good -- I ran my personal slowest 5k this past weekend. Granted, I didn't go all out, but it was at a significantly slower pace than my half-marathon PR from a few years back. I set that PR about a month out from a marathon, when I was logging 50+ mile weeks. I was doing pretty much zero speed work, even then. That race wasn't even full effort, since I was treating it more as a long tempo run than a race. My 5k PR was the first leg of a duathlon. I've yet to run a real 5k race when my mileage is high enough to see what I'm really capable of.