strategy is becoming more and more popular, even among the elites (either that, or Amy Hastings is spying on us). In fact, many people end up PRing in a 5k shortly after completing a marathon. Why? I mean, it's not like the "traditional" training for either of these races is similar (shorter, faster intervals are emphasized in 5k training whereas more tempo-based running finds its place in marathon programs). However, it's these longer runs that give you a head start on the rest of the 3.1ers, as they help your vascular system to be more efficient (increased capitalization, blood volume, mitochondria, and a stronger heart).
To begin the transition, you need to re-train your neuromuscular system to get used to shorter, faster intervals. Basically, you have to retrain your body to deal with stronger, more intense pain (at least it's only for ~1/8 as long as before). Try adding 8-12 100m strides after an easy run a few times a week or running up a steep hill for no longer than 15sec (per interval). This will promote an increased muscular variation and a faster cadence. After you get your legs back under you, try some 200's at your 5k goal pace, and then gradually increasing the interval distance (up to 800m). Tempo and long runs remain part of the schedule... just a little shorter and a little faster than before.
Note: neither marathon training or 5k training will help you out in a 5k marathon... that's a beast unto itself ;-)