Doubling and Hard Workouts
Shake-out runs, or getting in a few easy miles the morning before an afternoon workout or race, can be considered doubling. These runs can be anywhere from 10 to 20 minutes, and can be thought of like an extra "warm-up." Jogging these easy miles before harder efforts will make your legs feel fresher and more responsive during the later workout session.
Doubling in the reverse fashion—completing an easy run after a hard morning workout—flushes out some of the lactic acid build-up in your legs, and can aid the recovery process. If you run again on the same day as your hard morning workout, more than likely your legs will feel more tired than you're used to; you might question whether the second run is worthwhile. Of course you'll feel tired on the second run because you're still fatigued from the earlier hard session. The feeling "better" will come in the following days. The more you complete easy second runs the same day after a workout, the more your legs will adapt to the previously fatigued state, increasing fitness gains.
Double With Cross-Training
For runners who are either prone to injury or looking for a way to enhance their endurance and cardiovascular fitness without the possible injury risk of running extra miles, cross-training is an excellent solution. The benefits of a running and cross-training double are much the same as completing two runs on the same day but there's less pounding on the body.
If integrated gradually and you keep your cross-training intensity easy, cross-training double days will increase your endurance and overall fitness.
How often should you complete doubles? It depends on your level, goals and total weekly miles, but you can safely work up to five double days a week when your ability as a runner reaches the advanced level. The remaining days should be reserved for your long run (no double) and a day true recovery or rest day.
Double Day Integration
Clearly not every runner should start pounding it out twice a day, five days a week, right away. When you start adding doubles, track your total daily volume and, as you increase your training load, take special notice of injuries, pain or lingering soreness—if this happens, stop adding running miles and only add cross-training time.
Don't double on a long run day—you've already gotten the miles in and your body needs the extra recovery. You could complete a stretching or core session in the afternoon on your long run days if you want to do something extra.
Integrate double workouts gradually. Start by splitting up the miles on one of your recovery days, keeping the same overall total miles for the day. Next step: either add more double days—one extra double day a week after a couple of weeks to give your body time to adjust—OR increase the distance of some of one of your double runs. Over time, you'll see how your body responds, and you can keep implementing more double days if it's working for you. Again, if you notice extra tiredness, soreness, pain or an injury cropping up, scale your schedule back, or schedule down weeks where you take a break from doubles.
The other way to approach doubling would be to keep your running schedule the same, and begin adding a second daily cross-training workout. Once you've adjusted to doubling on your easy days, you can move to the hard workout days.
Regardless of the situation, doing more than one workout a day is designed to improve your fitness. As long as it's done intelligently, the more you can stress the body through training, then allow it to recover, the more results you'll see long term.race.