Running is a great way to improve the muscular endurance of your legs. And certainly, if you haven't been exercising, running will lead to muscular adaptations in strength to support your new running routine. Also, running hills or sprints will lead to further adaptations in muscular strength due to the challenges presented in these types of running workouts. That said, running is primarily an endurance activity. When your body has made the initial strength adaptations to support your running routine, it won't continue to grow stronger or develop more muscle mass. If you're looking to improve muscle strength and mass in your lower body, you need to include resistance training moves in your exercise routine on a regular basis.