Cycling Question? Not getting better :/?

Ive been biking my whole life, and have been biking everyday possible for the past year. I bike ATLEAST 5 miles a day, thats the very least. I bike for fun and transportation. I do a 10-30 mile bike ride at least once a week. Im 15 years old, and I dont seem to be getting any better at cycling. I can climb hills a little better than I used to, but stamina doesnt seem to be changing. Anyone know why this might be? I ride an SE lager single speed bike, and biking isnt the only exercise I get. I run everyday, and use bicycle machines in the gym as well.