Runners often start running twice per day before weekly mileage warrants it. Doing “doubles” sounds like serious training, so it must be better preparation. In specific situations, this is true. Most runners, however, should resist the urge to switch from single runs to doubles as training mileage increases. Let’s take a look at when double workouts are and are not beneficial, and how to add doubles to your training program.

A basic principle to follow is to not do double workouts until you have maximized the amount you can handle in single workouts. Staying with longer single runs builds endurance, while shorter doubles allow you to train at a faster pace. Double runs can also be beneficial in speeding recovery. Two short runs will help you recover more quickly than one longer run.

The weekly mileage at which you should add double workouts to your training schedule depends on the distance you race (see table below). The longer the race, the more your training should focus on endurance-based adaptations such as depleting your glycogen reserves to provide a stimulus for your body to store more glycogen, and training your muscles to utilize more fat at a given speed. Logically, you will provide a greater stimulus for these adaptations through a single 12 mile run than by doing a 7 mile and a 5 mile run at the same pace.

As shown in the table, if you are preparing for a marathon and are running less than 75 miles per week, then you should not be running doubles. After you schedule your long run and a mid-week medium long run, there is really no reason to double to get in the remaining miles. Once your marathon training calls for more than 75 miles per week, however, there is a definite role for double workouts in your program.

The shorter the race that you are preparing for, the lower the mileage at which you should add double workouts. If you are preparing to race 5 km, for example, your interval workout is the most important training session of the week, and you will need to keep your legs fresh. You will also want to maintain a faster pace during some of your normal training runs, which is accomplished more easily during two short runs than one longer run.

When to add double workouts

Race distance prepared forWeekly mileage exceeds
5 km or less55
8 km to 12 km60
15 km to 25 km70
30 km to marathon75

How should you introduce doubles into your training program?

Like any other aspect of training, doubles should be introduced gradually. You should start by adding one double per week, and then another, as you gradually increase your mileage. Those hardy souls running more than 120 miles per week will likely reach 4 to 6 doubles per week.

The minimum time for an added second run should be 25 minutes. If you run less than that, it is hardly worth the extra time and effort both physiologically and in your busy life to change and shower. Also, in some situations it will be wiser to increase your aerobic training by adding cross training to your program rather than increasing your risk of injury with more miles of running.

Start adding doubles as easy second runs in the following order of priority. As your mileage goes up, add doubles on days when you do: 1) long intervals or speedwork; 2) tempo runs; 3) races of 10 km or less; 4) recovery runs; and 5) medium long runs. You should also follow roughly this order for adding second workouts of cross training, unless the cross training is being done specifically for injury rehabilitation.

An easy run in the morning will loosen you up for an evening session of long intervals or speedwork. Similarly, 30 minutes of easy running in the evening will help you recover from a morning tempo run, interval workout, or tune-up race (note: If you have raced more than 10 km, then your need for rest will definitely outweigh the benefits of a recovery run).

When your mileage increases to the point at which your recovery days call for 8 miles or more, then it is time to switch those days to easy doubles. It is easier on your body, and your recovery will be enhanced, if you do two runs of 4 miles rather than a single 8 miler. Please avoid the trap of adding mileage to your recovery days for the sole purpose of boosting your weekly mileage. Extra mileage on these days is counterproductive in that your recovery is less complete for your subsequent hard days.

Finally, an easy 30 minute run on the same day as your medium long run will provide an incremental training stimulus by depleting your carbohydrate stores and training your muscles to rely more on fat at a given speed. It is preferable to add 30 minutes in the evening after a medium long run in the morning. Only do a morning run before an evening medium long run if it will not tire you out for the evening run. A better quality medium long run is preferable to a double in which your medium long run is a slog.

Definitely do not do a second run after your weekly long run. I tried this a few times and it is a perfect example of mileage for mileage’s sake. An evening run after your long run will only slow down your recovery. As soon as you finish your long run your objective switches to recovering as quickly as possible for your upcoming hard training days.

(This column originally appeared in Running Times Magazine.)