(Who knows? I might even be ready to race by the end of the year.)
I watched Michael Phelps doubly-humbled in what was once called his signature event, the 400-meter individual medley; first barely qualifying eighth, then finishing a distant fourth. Yes, Phelps swam the event many times, a hiatus after 2008 only foreshadowed Ryan Lochte's time to be the big man in the pool, As Michael walked away, looking much like a guy just released from a two-hour calculus class, it was plain to see a parallel to running.
The law of specificity says, in so many words, success at a particular activity requires focus on that particular activity. Weeks of elliptical trainer workouts and spinning classes have (some) cardiovascular and musculoskeletal benefit, but to run quickly in races, one must learn how to run quickly. Naturally, that can only come from running quickly.
Is "quickly" different for distance races beyond 10K? I first would say reactively there is a different form of quickness. But look a little closer at world class marathoners; those runners whose race pace are in the range of 4:40 to 5:00 per mile. That's not their training pace for every single workout.
There are a couple of different schools of thought when it comes to run training:
Grind out long runs at a sustainable pace and benefit from the slow but steady increase in speed which comes from enhanced aerobic base. Of course, this "long distance speed" comes at the increased risk of an overuse injury, as well as the loss of the "short distance speed."
How can a runner tell they've lost the short distance speed? That's when they show up at a 5K and run a solid thirty seconds slower than they did the previous year, BUT...they could most likely run the race a second time and duplicate the pace.
Want to keep all those "gears" in the "gearbox," especially if you like running distances other than the marathon? It's going to take some variety in training pace.
For the marathon I believe the training focus is ideally on increasing the pace where the aerobic threshhold ends and the anaerobic threshhold begins, Since the race distance is long(er), focus one workout a week either on cruise intervals of no less than a mile, or tempo runs of up to 20 minutes at approximately 10K race pace, each with brief recoveries.
Every two weeks, just to add a little variety, do some 200-meter or 400-meter efforts at 5K race pace, with 400-to-800-meter recoveries. I wouldn't do 80 of them, like Emil Zatopek once did in a single workout, but perhaps up to three miles worth might do.
So if you want to lower that marathon time, you're going to have to get that lower time down during the training. Slow and steady isn't going to cut it.