I have completed and trained a number of people in several High-intensity interval training (HIIT) programs.
Depending on what you are training for or what your goals are only training using HIIT programs may meet your needs. The programs I have used, and my clients use, we train 6 days a week about 30 minutes a session. A sound protocol should include body weight exercises that work a range of muscles, and provides some variation across each day within the program to keep your mind and body interested and always working new muscles / muscle groups.
Just because you are not weightlifting or running does not mean you cannot incorporate strength training type exercises within your routine ( think push-ups, pull-ups, planks, plank-walks etc.. )
HIIT programs are an excellent way to get a great full body workout. 15 minute sessions is far better then nothing, I personally would shoot for a bit longer but it sounds like you are doing great.
Keep up the great work!