Our 21K - 13.1 mile - half marathon pro run trainer improves your cardio, endurance and overall fitness with an awesome run training program. Start with 10K running and gradually increase to a full 21K ( 13.1 miles ) on your way to a full marathon. ★ You can do it! ★ The Ultimate Half Marathon training app, from the makers of the award winning health and fitness workout apps like Pushups, Situps, 5K Runner, 10K Runner, Squats and more apps. 21K Runner is the easiest and most successful 1/