My maternal grandmother—Mama Henry—made the best turnip greens. She fixed them like none I've ever had since. Somehow she cooked, drained and then chopped them fine. Then she cooked them still more in an iron skillet with bacon drippings. Yes, I know, doesn't sound very healthy, but it was the late 60s and I was only in kindergarten. Mama Henry passed away when I was in 2nd grade. And to this day, I still don't know exactly what was in those greens, but whatever that magic ingredient was, it made them pretty darn tasty.
And then there was my paternal grandmother—Mama Mac—who was famous for her pound cakes. A slice of her pound cake buttered and toasted with a dollop of vanilla ice cream on top was pure heaven on earth. Mama Mac believed that if it was "real" (i.e., whole milk, eggs, cream, sugar, etc.) then it was good for you. Growing up, we didn't have Mama Mac's pound cake all the time, but when we did it sure was good. Mama Mac passed away when she was 82; I was in my late 20s. We had her pound cake recipe, but no matter how hard different family members tried, no one could match a Mama Mac cake. One day, a cousin found Mama Mac's "real" recipe for here cakes. Basically all the "good" ingredients were doubled (8 eggs, 4 sticks of butter, 2 packages of cream cheese). That was her magic ingredient. I think a whole lot of love for what she was doing when she made her prized pound cakes was a key magic ingredient too. That kind of magic you can't replicate.
There are magic ingredients for marathon training too. Just like my grandmothers had different magic ingredients, runners will vary in their marathon training magic ingredients too. For many years, my marathon training consisted of weekly runs of no particular pace and then the weekend long run. Later I discovered that throwing in some speed work such as intervals really helped my endurance as well as upping my lactate threshold and VO2Max. But for me the real "magic ingredient" for me is the tempo run. In my old training method, I always wondered how (if you wanted to run a certain pace, but never ran that pace or faster in training) were you going to maintain that race pace on race day. Adding Tempo runs to my training helped answer that lingering question.
I think it's important for your body to experience running at faster paces if you're going to be running a faster pace in the marathon. Don't get me wrong. Your long slow runs, are still needed. They are designed to build your endurance both muscular and aerobic in order to run those 26.2 miles. Tempo runs, however are a nice blend between really fast speed work such as intervals and your long runs. Tempo runs typically begin with an easy-paced warm up mile and then the pace is ramped up to near 10K pace for the next 4miles or so before slowing it back down for the last cool-down mile. Hal Higdon also advocates using more of a timed method kind of like extended fartleks where you run easy for 10mins, then 10K pace for 10 mins, then easy for 10 mins, etc. So a 40-minute tempo workout would look like 10+10+10+10 = 40. Before the dawn of the Garmin and GPS devices, this timed method worked really well.
Tempo runs (like intervals at the track) are a great way to up your lactate threshold—the point just before lactate acid builds-up in the bloodstream causing fatigue slowing you down—that tell tale burring in your muscles. If you can push that threshold out, then you'll be able to go at higher intensities longer. Also, if you need to pull out all the stops later in a race, you'll be better equipped to do so.
Another magic ingredient that I've found helpful is running at race pace. The long runs have me running well below race pace and the intervals and tempo runs have me running well above race pace. I found it odd that I'd not been giving my body any experience running at race pace. There's no need to do a full long run at race pace, but I've found it very beneficial to take the last few miles of a few long runs and gradually increase the pace so that by the last mile or two I'm running at race pace. I'm a strong believer in simulation. If you're able to simulate any or all of the races condition (temp, precip, course, pace) during your training, you'll be better acclimated and conditioned for race day.
Like I said earlier, everyone is different. What works for me may not work for you and vice versa, but one thing I've learned over the past 25 years of running is that what works for someone else might just be worth trying...at least once. So, what's your magic marathon training ingredient?