Higher volume runners practice doubles, where they add a 2nd shorter run later in a day after a prior regular morning run.
It’s a key to building those 120+ mile weeks that elites run. Otherwise, such a runner’s typical workout tops 10 miles and with few exceptions that’s not sustainable long term.
However, miles on your legs are still miles on your legs, and a runner wanting to avoid burnout and injury probably should avoid two runs on easy days.
Still, there’s value in endurance training with doing double workouts, and there’s an easy way to do two workouts in a day without taxing your legs through an extended run more than once.
Just cross train for the second workout. It seems so obvious, and yet so many don’t think to do it. Cross training is low impact aerobic exercise, and there’s a reason IronFit refers to the practice as “Free Miles”. Even if you’re not actually running, you’re working and developing aerobic fitness that will help you down the line.
On top of that, you’re resting bones, joints and muscles that have to do work on a regular run, and avoiding wear and tear that exacerbates the amount of recovery you need.
For example, you run 6-10 miles in the morning. You go through your workday. After work, instead of a 3-4 mile recovery run, you hit the spin bike for 45 minutes at an easy aerobic heart rate. Or you use the rowing machine for half an hour. Or the ARC Trainer, or the elliptical. You get the idea.
You could also do strength training for that 2nd workout instead, provided your body is up to doing so. The extra anabolic boost could jump start your overall recovery, especially when paired with a good healthy dinner and a lot of sleep.
Basically, there’s no law stating that to do a double workout day your 2nd workout has to be another run. Provided that morning workout was a full aerobic run, you could do just about any other form of cross or strength training for that 2nd workout and still receive dividends.