The bridge you're looking for is a tempo run. The best definition of a tempo run that I've seen is running "comfortably hard." That means you're running faster than your normal pace, but not quite at your race pace. As a veteran marathoner, I've always wondered why the long run was always done at a slow pace. I understood the reasoning of building endurance without increasing the chance of injury, but in the back of my head, I always wondered how my body was all of the sudden on race day going to "kick-it-up-a-notch" and run at race pace. Tempo runs help address that dilemma.
Basically the tempo run does two things. For one, it helps mentally by allowing you to run some longer distances near race pace. This helps build your confidence by bridging that gap between the really fast intervals and your slow long runs. Secondly, it helps train your body to deal with the by-products that come with running faster and longer. Have your muscles ever become fatigued during a marathon race? (Stupid question, huh?) Well, this fatigue (in part) is due to lactate and hydrogen ions that are released into the muscles as a by-product of metabolism. This increases the acidity of your blood which fatigues the muscles. So how does a tempo run help with this?
Well, part of the reason lactate is a problem is that most runners haven't trained their bodies to use the lactate to their advantage. Yep, you heard correctly. Lactate can be a good thing. It can be used as an energy source when your body shuttles it to the liver where it's changed into glycogen (fuel). Problem is that most runners don't push themselves in a long run until the race and at that point, the body doesn't have time to adapt and figure out how to use the lactate to it benefit, so all the runner experiences is fatigue. A tempo run helps to increase your lactate threshold. In other words, you can actually train your body to push that threshold helping your body learn how to use those by-products to your muscles' benefit which means less acidity of the muscles, which translates to less fatigue. Basically, that's a lot of scientific garblety-gook for...throwing in some longer faster runs will help you be a more efficient runner on race day.
If you're training for a 5K, adding a weekly tempo run of 2-3 miles will do the trick. If you're running a 10K, add 4-6 mile tempo run during your peak training weeks. Running a half marathon? Do a 6-8-mile tempo run during your peak weeks. Marathon? Throw in an 8-10-mile tempo run during your peak weeks.
The FIRST marathon training plan actually varies the pace a little during its tempo runs. It incorporates four different levels of tempo runs—marathon pace, short, mid, and long. The marathon pace tempo run is exactly what you think; running at marathon pace. The short tempo is run at about 15 seconds slower than 5K race pace. The mid-tempo is run at a pace approximately 30 seconds slower than 5K race pace and the long is run about 45 seconds slower than 5K race pace.
Also, keep in mind that you should begin a tempo run with a 1- to 2-mile easy warm-up that leads right into your tempo pace and distance. Some plans have you also slow down the last mile as a cool-down. So, a 4-mile tempo run may actually be 6 miles total running—1mile warm-up, 4-miles at tempo pace, 1-mile cool-down.
Take a look at the video clip below from American long-distance runner, Ryan Hall, as he explains the importance of the tempo run.