There a tons of benefits to running a 5K or marathon—improved cardiovascular fitness, more stamina, working out for a good cause if you run for charity—but seeing the number on the scale go down isn't necessarily one of them. All the training you do to cross the finish line makes your body efficient at conserving energy so you can go the distance, and as you increase endurance, you’ll gradually start burning fewer calories during your runs, says Jon-Erik Kawamoto, a certified personal trainer, strength coach, and former competitive runner. Great for your race, but the exact opposite of what you need to lose fat. Couple that with the common increase in appetite—and subsequent increased calorie intake—and some runners may in fact gain weight.
To meet your race goals and shed a few pounds in the process, supplement your running program with up to three resistance training workouts a week that focus on equally working opposing muscle groups (such as your back and chest) and improving joint mobility and function to build strength and burn additional calories, Kawamoto says. He also suggests switching out one day of running for a cross-training cardio workout to help prevent injury and offer a new challenge to your cardiovascular system. And don't forget to be sure your eating plan provides the nutrients your body needs without adding unnecessary calories.