Bit depressed,
Apologies, but I too have had a chance to get some sleep, and reconsider your situation and my earlier advice. First, you might still be wise to incorporate some regular strides (say 1-2 x per week) and maybe also a 4-6 week phase that includes repetitions (a la Daniels)...
However, I apologize for not looking closer at your current times. And, considering your overall mileage & ultimate (only?) goal of a sub-3 marathon.
I think more miles will help you a lot. For several reasons.
1) 40, 50, miles is not much to maximize marathon performance.
2) Your endurance appears weak. Looking at the Daniels charts, a 19:00 5k equates to a 3:01:39 marathon. Your 3:17 is more equivalent to a 20:39 5k. And, a 6:03 mile. You may feel like you are weak on the speed-side, but your times suggest you are weakER on the endurance/stamina/aerobic side.
Here is an article that might be enlightening, and show you the importance of overall mileage. Make sure you actually do the math for the different levels for your 10k and see the difference.:
http://www.hillrunner.com/jim2/id64.html3) From my own experiences of running in the 40-60 mpw range, and eventually getting to the 70-90 range, I can 99.999% guarantee you that if you follow a well-rounded plan and increase your mileage, your 5k time will improve as well. At 40-60 with plenty of speedwork, I was struggling to break 17. Now, in the 70s & 80s, and focusing more on the half marathon than the 5k, I am disappointed if I'm slower than 16:00. I've cycled my mileage up & down enough to know that the mileage is the main difference, as long as you aren't completely neglecting some faster paces (again, well-rounded plan is important)
4) Weight - do not underestimate the importance of this. I spent years training hard, and running with 10-15 lbs more than I should have been. Extra pounds matter. Easily 1-2 seconds per pound per mile. At 1.5 seconds, every pound is about 39 seconds off your marathon. This article opened my eyes:
http://www.serpentine.org.uk/pages/advice_frank02.html**Matt Fitzgerald cites some research in his book Racing Weight, that suggests running higher mileage somehow signals your body to seek a lower weight. Run more miles, and eat good foods sensibly (i.e. don't "diet" & deprive your body the fuel it needs to train well), and with time you'll be pleasantly surprised with the changes that happen in your body. There might be something to his theory - at lower mileages, I have not had any success trying to hit my goal racing weights via dieting & deprivation, but with higher mileages & eating good foods I did. Who knows, but worth a try.
Sorry for the change in advice, but I didn't want to stear you in the wrong direction. You seem to have a lot of opportunity on the endurance/aerobic side. As long as you have the willingness & time in your life to put in more training, I bet you can get your sub 3 this way.