If you break up your long run, you won't deplete your glycogen fuel tank, so you'll miss the opportunity to make the adaptations. Long runs also prepare your muscles, bones, tendons and ligaments to handle the stress of running on rugged trails or hard pavement, and callous you for the psychological fatigue that accompanies running for a long period of time.
Running Doubles Boosts Your Hormonal Response
Many of the adaptations you make from training are initiated by signals from hormones, which are amplified and transmitted via signaling cascades, and lead to protein synthesis. This signaling is fast, occurring within minutes of completing a workout. When you run twice per day, you get two hormonal responses and thus two opportunities for adaptation to training because you have more frequent signals for protein synthesis.
Repeated hormonal responses lead to a concerted accumulation of structural and functional proteins in your muscles, like more mitochondria and enzymes, improving your aerobic fitness and enabling you to run faster.
Running Doubles Helps You Lose Weight
If you're interested in losing weight, running twice per day can be an effective strategy. For one thing, it allows you to raise your daily and weekly mileage, which increases the number of calories you'll burn. Running also elevates your metabolic rate for a few hours after you stop, as your body uses fat as energy to recover and return to homeostasis. Splitting your run into two shorter runs gives you two separate elevations in your post-workout metabolic rate, which gives you two opportunities to burn more calories during the day.
A study published in British Journal of Sports Medicine had women run for 50 minutes at 70-percent VO2 max one day and twice for 25 minutes at the same intensity another day. Another study published in Canadian Journal of Applied Physiology had men cycle for 30 minutes at 70-percent VO2 max one day and twice for 15 minutes at the same intensity another day. Both studies found that the combined increase in post-workout metabolic rate was higher after the workouts, compared to a single workout.
5 Strategies for Running Doubles
1. Run very easy during your morning run.
2. Allow at least five hours between your workouts to recover.
3. Consume about two calories of simple carbohydrates, like glucose, per pound of body weight (0.5 gram/pound) immediately after your first run to speed recovery. For a 150-pound runner, that means consuming about 300 calories of carbohydrates, which equals about three 8-ounce glasses of chocolate milk. Keep consuming carbs over the next few hours.
4. Rehydrate after your first run by drinking 15 ounces of water or electrolyte drink per pound of body weight lost during your run.
5. Start by doing two-a-days once or twice per week, cutting the length of the single run you'd normally do by 15 minutes. Don't do more than 10 to 11 runs per week. For example, if you normally run 60 minutes, cut that back to 45 minutes, and do a second run of 30 to 40 minutes.
The Benefits of Running Doubles
Many of my athletes training for long races have benefited from two-a-days when running high mileage. They say it keeps their legs feeling fresh while still accumulating a lot of miles. When they run more than an hour per day, it's harder for them to bounce back the next day for a workout. Any time I have coached a runner who runs more than 50 to 60 miles per week, he or she feels a lot better when breaking some of the runs up into two runs per day. You'll notice a difference, too.
So, next time something unexpected happens that causes you to cut your run short, don't fret. Just break up your run into two. It may be just the double boost you need.race.