Why am I not getting any fitter?

Hi, I'm a 17 year old guy, and since christmas I've been exercising quite a bit, 1 Just so general health will go up and 2. to lose a bit of weight. I'm not really fat, but too chubby. My routine now is I try to run 4 - 5 times a week, usually about 2 - 3 miles and on each day I run, I do some weight training. The thing is, I'm barely getting any fitter, I'm still usually really tired after 2 miles and I'm not sure why. I'm naturally a big build, and (without meaning to boast) I'm very strong strong, and my legs and arms are big, with barely any fat on them, due to my large build am I just not built for long distance running or something? Should I be focusing more on weight training? I know it can take time to see results but I've been running for over 6 months.