Need to improve my running from HM to full marathon for May 2014

Congrats on your first HM.  Pretty good time too if you ask me.  You definitely have some running potential to pull that kind of race off on 20-25k a week.  That said, a marathon will likely require much more running volume to keep running towards that potential.

My advice would be to start building a real running base now.  Look for something like the BarryP 3-2-1 plan.  You can search for some good threads discussing it on this forum and on Slowtwitch...or you can just google it.  It basically outlines a good way to build up your running safely by running often.

Once you've built up a good base...maybe start looking for a marathon training plan around late January.  Look for a plan that starts at the running volume you are comfortable with at that time.  There is really no point in trying to pick a marathon plan now...as you may be behind or ahead of the starting point of that plan when the time comes.

Lastly, while I think you are a good BQ candidate, don't let it drive your training.  Run the volume and paces that you can comfortably handle while avoiding injury.  If you eventually get to BQ in your first marathon, that would be a great accomplishment.  Maybe do a few 10ks and another half marathon to help guide your fitness progress and training paces.  So many people become fixated with a BQ, or sub X:XX marathon goal that they push themselves way too hard considering where they are starting.  Then they end up injured.

I think you're contradicting yourself a bit.  You mention slowly ramping up mpw, but then lay out a weekly schedule that is 3x more than what you currently stated your running.  That's likely a recipe for disaster.  Use your currently weekly mileage as a starting point, and increase from there - most will suggest to stay within the guidelines of 10%/week, and try to keep the long run to 25-30% of total mileage. 

The other two key peices are consistency and pace.  Pace while you're in this base phase is going to be mostly easy - which means much slower than your intended marathon pace (even for the shorter runs).  This can and will change once you move to a specific plan, but the easier pace will allow your body to adapt to the increased mileage.  It's not easy to run slower, but there's lots of evidence to show it's a safe effective way to build the mileage, and allow your body to adapt.  Use the Mcmillan calculators to figure out your pace - you'll be shocked how much slower you need to run.

Jason suggested it before, but take a look at the BarryP program. 

Yes.  Just because you can currently run 20k at an easy pace just fine now (when your overall mileage is low and inconsistent), it doesn't mean you will be able to safely maintain that long run while increasing your overall mileage and consistency by adding other runs.  The cummulative fatigue and training stress you build up from all your running is real, and you shouldn't put that much stock into your long run.

If you read through the BarryP plan, it basically asks you to take your current weekly mileage and break it up over 6 runs, with 3 short runs each at 10% of your total mileage, 2 medium runs at 20% of your weekly mileage, then 1 long run at 30% of your weekly milege.  See how the 3-2-1 ratios are implemented?  Simple right?

So for you, even though you haven't been consistent at it, I'd say you could probably start off at 30k per week.  Which means three 3k runs, two 6k runs, and one 9k run.  I know, I know...that sounds ridiculously low, but your initial gains will be quick.  The key here is building up consistency and allowing your body to get accustomed to higher volume in a safe way.

You can probably get away with adding a little more than 10% for the first few weeks, but once you get towards 40-45k you probably want to cut it back down to 10%, or maybe even 10% every other week.  You don't have to increase every week.  The key is to only increase when you feel like your current running isn't providing enough training stress.  Sometimes you may not need to increase for 4 weeks because you're still getting accustomed to running your current volume comfortably.  You can also choose to increase the intensity of your mid distance runs instead of increasing distance...but you probably shouldn't increase both distance and intensity at the same time.

Improving running fitness takes patience.  Especially if you want to do it while avoiding injury.  For most people (myself included) it's a "hear me now, believe me later" process.

I don't think burn out or injury is a great way of judging if you running "enough"