Rest days are important, but how much do they matter for running? Beginner programs like couch to 5K often start you off with three days of running per week. If you get antsy on the days in between: Yes, it’s OK to run more. With some caveats.
The main thing to remember is to build yourself up slowly. Nearly all the common beginner injuries stem from running more than your body is used to. If you just started a three-days-a-week plan, stick to it for a while.
But eventually ... yes. After you graduate from couch to 5K, feel free to add a fourth day, then a fifth and a sixth—but do it one at a time, making sure your body has adjusted before you take the next step. Pro runners with years of experience will often hit the road or track twice a day to get in all their miles. That’s OK if you’ve worked up to it.
It’s helpful to think about your weekly mileage as a total. If you ran 16km over the course of last week, it’s probably fine to up it to 12 for next week, but not to 20. (The rule of thumb is to increase by 10% each week, but you have some leeway to play with that number: everybody is different.) Bottom line: Run as often as you like, as long as your body is prepared for it.