When you’re gearing up to run a half marathon or marathon—jogging upward of three or four hours at a time—fast workouts might seem like the last thing you should be focusing on. After all, even if you hit your goal pace come race day, it’s nowhere near a sprint.

But adding just one quicker-paced session to your weekly routine could give you a big boost at that starting line. “Speed work is important, no matter what distance you’re pursuing,” says Deena Kastor, Olympic bronze medalist and the American record holder in the marathon. “Running fast keeps your stride efficient at all paces, which prevents injury and also makes running at slower paces feel like less perceived effort.”

“Speed work is important, no matter what distance you’re pursuing.”

The key to effective speed work is to dial into certain distances for your quick miles, depending on which race you have coming up. The fast-paced runs you do while prepping for a long-distance race should look very different from those if you’re about to run a 5K or 10K, for example.

For those shorter races, speed work is all about short sprints and hill repeats to train your body for quick bursts of power and efficiency over a shorter distance. When it comes to longer distances, though, those sprints aren’t nearly as necessary. “For marathons, mile repeats and tempo runs are what’s important to support training,” says Kastor. “The marathon is a fat-burning event that requires you to run aerobically,” she explains. “You need to train your aerobic system by progressing your long run. The speed work simply improves your efficiency at running long distances.”

Kastor recommends adding both mile repeats and tempo runs (AKA threshold runs) to your schedule. The mile repeats will improve your efficiency and get you ready for the tempo sessions, she says. The tempo runs build on that efficiency and train your muscles to deal with higher levels of lactic acid—helping you hang on to race pace for an extended distance.

Get the speed workout in on Tuesday or Wednesday, then save your long run for the weekend. That way, you’re not tackling two runs that are tough on your body on back-to-back days. And here’s some extra-good news: When you add speed to your schedule, you also get to add more rest to give your body a chance to recover from the added strain you’re placing on it, says Kastor. (She advises taking an additional day off or getting into bed 30 minutes earlier each night.)

Here are Deena Kastor’s top speed sessions for long-distance training. Do one per week, alternating between the two workouts.

Workout 1: mile repeats

Warm-up: Jog at an easy pace for 15–20 minutes.

Work out: Do 3 or 4 mile repeats at a hard pace, resting for 2 minutes between each. Don’t go out too quick—think about getting progressively faster with each mile.

Cool down: Jog at an easy pace for at least 15 minutes.

Workout 2: tempo run

Warm up: Jog at an easy pace for 15–20 minutes.

Work out: Run 4–6 miles at a comfortably tough pace. (Aim for 30–40 seconds slower than your 5K pace; you should be able to speak just a couple words at a time.)

Cool down: Jog at an easy pace for at least 15 minutes.

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