Research shows it’s not just how much you run, but how hard you go that plays a key role in improving health and performance. Even small amounts of higher-effort activity added to your routine can make a meaningful difference. When it comes to boosting performance and health, many runners tend to focus on frequency and duration of activity—for example, how many days a week you run , along with total weekly mileage . But there’s another variable that is just as important when it comes to progress and disease prevention: intensity . A new study published in the European Heart Journal , looking at data from about 475,000 people, found those who incorporated vigorous physical activity into their regular routines were at the lowest risk of developing major chronic diseases.…