Which "systems" do I need to develop

Not sure how much rest you got with those mile repeats, but based on that alone you should be able to break 18 for a 5k already. I don't think your size has anything to do with it, just run more. More distance, in general. Along with that, im sure you're weight will drop slightly too.

Your aerobic and anaerobic systems are fully developed. Your heart and lungs will not develop any more, nor do they need to. What you need to work on is speed endurance.

How much body fat are you carrying? A lot of runners have a lot more than they realize. Don't believe the low figures that are often quoted, they come from innacurate testing methods. The reliable method is the skinfold caliper method. A 10 per cent reading from this method is actually very low body fat, but how many athletes realize this? They think that is not a low figure.

1. While I don't think you are underdeveloped, your lungs and heart do become more efficient as you run more.

2. Being honest, I think your 5k could be looking closer to 17:00 than 18:00. If I'm running mile repeats in 5:20, I'm looking closer to 16:40. However, I'm giving a bit of leeway here since you might have been hammering the miles instead. Your half isn't too bad, but if you are looking at sub 18 for 5k, 1:20 for the half shouldn't be too hefty of a goal and if you are only able to run 1:25, then it is truly safe to say that you are underdeveloped aerobically.

3. You've got a summer ahead to drop 10-15 lbs if possible from running alone. That can help any distance outside of the 100-400 range tremendously, especially the longer you go.

If you're a muscular 185 I'd leave it alone, if not cut the calories and drop to around 140-150.

Also that is your weekly training mileage? If you're marathon runner you should be running once a day 6 days a week for 60-70 mpw.

Tell us about your training. As a HS state placer in CA at 215, your optimal event is most likely the mile and training a natural miler to run the marathon and not hit the wall is not easy.

I'm guessing here because I don't know what a typical training week looks likes for you, but I think you could probably benefit from more threshold work to get you stronger and better able to hold a fast pace for longer. So I'd suggest making a tempo run the focus workout of your week for awhile and see what happens. The workout can vary too, i.e., you can do a traditional 5 mile tempo at your 10 mile/half marathon pace, or you can turn it into a progression run, or you could break it up into something like 2 x 15 minutes or 3 x 2 miles. My other suggestion would be to gradually increase volume after you get adapted to the tempo run (unless you're already doing them).

If you're doing 3 x mile @ 5:20, your goal should be breaking 17, not 18 in the 5k. I think sub 1:20 is a good goal for the half and 2:55 for a full.