How can I build up the number of days I run per week?

I'm sharing what I used to follow in my early days of running (and still holds good):

• Time it right: Try it when you have no races coming up, or early in a new training plan. ‘This gives you the opportunity to experiment without ruining race preparation,’ says Gaudette.
• Test it: Start by adding a short, easy run – about half the distance of a typical easy day for you. Injuries, anxiety, or bad sleep should prompt a return to your previous schedule.
• Assess it: After a few weeks, take stock. Maintain the frequency if you feel good, but scale back if you note signs of overtraining such as fatigue or slow performances.
• Step it up: Once you know the extra day won’t break you, add a mile every two weeks until you match your other easy days. Then you can add short bursts of faster running if you like.

Calculate what 10 to 15 percent of your goal weekly mileage is, then increase your mileage each week by no more than that amount. For example, if your goal is to run 60 miles per week, you should increase your mileage each week by no more than six to nine miles. If you usually run 10 miles per week, increase by no more than 1 to 1.5 miles the following week.