>Running 101>How to improve my speed
You mean that from the 15 mile marker on? Yeah, I would chalk that up to minimal mileage, and you would definitely benefit from a higher mileage plan, probably more so than introducing speed work at this point. The cycling and swimming are good cross training.
The Logic of Long Distance
Scout7 is right. Higher miles will help, but you might also add 6-8 60m strides at the end of your runs 2-3 times per week. The strides should be steady accelerations to an almost-all-out sprint (no straining). Be careful the first few times you do it so that you don't pull a hammy.
Another workout I like requires a track. The idea is to run 10x400 continuously at (current, not goal) marathon pace, but run the last 100m of every lap as a controlled stride, accelerating to the top velocity good form will allow and holding it for 100m. So, it's 10 continuous laps, 300m at marathon pace, 100m fast. I find that this helps me to develop and maintain different gears--and also practice recovering at marathon pace. You might not start out with 10 of these--try 5 or 6 the first time and build.
These are two things you can build into your training to work specifically on your speed without sacrificing your ability to build the aerobic base that is everything in the marathon.
I would say, though, that before you start introducing those workouts, get your consistency up to about 5 days per week.