Miles, miles, miles
If you only have an hour to run and shower before heading out to work in the morning, then a double run can help you push up your weekly distance - and slowly and carefully increasing your weekly distance is the single best way to get fitter.
One study found that the total distance run in training predicts up to 59% of performance score variability among world class long-distance runners. In layman's terms, the fastest elite runners tend to be those who run the most.
The same rule also applies to amateurs and across various distances. A 2020 study showed that running over 32km a week and completing a race-length long run in training was associated with faster half marathon performance.
As ultramarathon coach and exercise physiologist Sean Bearden puts it: mileage matters most. So fitting in a second easy run in the evening to add some extra miles can be a really valuable addition to your training. The key is to not increase your mileage too fast in order to minimise injury risk: legendary coach Jack Daniels suggests ramping it up by no more than 10% every 4 weeks.
But it seems that double runs aren’t just a way of getting in more mileage - they actually have more specific impacts on training.
Run faster for longer
There isn’t a huge research literature on double runs specifically, but what papers do exist have mostly been co-authored by David Bishop of Victoria University, who you can listen to discuss his work at length with Sean Bearden on the Science of Ultra podcast.
The basic summary is that his research suggests that training twice in one day might have beneficial impacts on both mitochondrial biogenesis and the rate of fat oxidation.
What does that mean? Well mitochondrial biogenesis is the production of new mitochondria in your muscle tissue. The more mitochondria you have in your muscles, the more energy you can produce at easy and moderate paces whilst running. This is one of the key physiological adaptations we gain by training, so an extra boost is always welcome.
Your rate of fat oxidation is your ability to burn fat as a fuel source. So an increase in fat oxidation at a given level of effort means that you conserve more carbs later into a race, thereby staving off the dreaded bonk - when you run out of carbs entirely and start running on fumes.
In combination, twice a day training could then be expected to be part of a training programme that produces runners who run faster for longer compared to runners who only train once a day.
Implementing double runs is easy enough. Eat well throughout the day so that you’re not totally depleted before you head out for the double, then lace up and run really easy. That means between 70-80% of your heart rate max, at the kind of level of effort where you can still talk comfortably in full sentences. It doesn’t matter if it’s slow - it’s your second run of the day, after all. These miles are all bonus miles. You’ll want to keep the distance short at first, probably around 50% of what you ran on your morning run.
Enjoy not being under any pressure and just stretching your legs. Running doubles won’t change you into a champion overnight, but it can be a great component in a wider plan.
One last thing
In the South of England, there is a very special fortnight-long window at the end of February when running at 7am and 5pm means you run at both sunrise and sunset. For those two weeks, you won’t care about what it’s doing to your mitochondria - you’ll just love double running.