I went from 38.09 for 10K to 34.36 in just under a year, but the first was relatively early after I'd started running again after a long, long break. During the next year it was just a question of training reasonably regularly, building up to 30+ miles per week. Like BR, it's covering the miles, week in week out, that I think played the biggest part in the improvement, but also doing long intervals (i.e. 6x1K, 5x2K, etc) along with some slightly quicker stuff towards the latter part.