By running more often, you reap the same fitness benefits you get when you boost the duration and intensity of any one run: reduced body fat, increased VO2 max, and improved muscle tone. You just get those benefits sooner.
I definitely felt the advantages of running twice a day. My morning run was easy enough that I wasn't exhausted in the afternoon. But I wasn't fresh either, so it got me used to training through fatigue. I always rehydrated, rested, and refueled before the afternoon run, so I had a much better chance of hitting a quality pace than if I'd tried to grind out those same miles on the end of one long run. Here's how to make doubles work for you.
Build SlowlyStart by doing two-a-days twice a week. Initially, the extra workout can be 20 minutes. When you first add it on, drop the length of your main workout by 10 to 15 minutes. As you get more comfortable, bring the main workout back to its original level and extend the first run to 40 minutes. You can double up on as many days as you want. Just spend at least two weeks at each stage before adding more miles.
Recover RightAllow at least four hours between your workouts so you can recover fully. After the first run, be sure to rehydrate, and consume at least 500 calories within 30 minutes of finishing to help speed recovery.
Mix and MatchStay alert for aches and pains, and fiddle with the formula until you find what works best for you. Many of my runners prefer to add a run in the morning when they know they have a hard workout in the afternoon. The easy run gets them loosened up for the hard work later. Others prefer to add extra workouts on their easy days because they're not as fresh after the morning run. Try some of the combinations below.
Two TimingEach of these paired workouts has its own unique benefits.
A.M.: 3 to 4 at easy aerobic base pace
P.M.: Interval workout (e.g., 6 x 800)
PAYOFF: Provides extra aerobic work without fatigue, gets blood flowing through muscles
A.M.: 4 to 5 miles at easy aerobic base pace
P.M.: 4 to 8 miles at a conversational pace
PAYOFF: Improves running economy, boosts weekly mileage, and provides a good fitness base
A.M.: Long run; or 5K or 10K race
P.M.: 2 to 3 miles at a conversational pace
PAYOFF: Increases blood flow to muscles, flushes waste from muscles, and speeds recovery.