If you don't have a bike, get one and start using it to go everywhere instead of motor transport. You'll soon be fit.
When I was your age I used to do a 3 mile paper round starting with 2 satchels of papers, then ride to school about 3 miles. I could never eat breakfast once I hit puberty (usually still can't) so the upshot for my body was that I couldn't put an ounce of fat on - whatever was excess calories from what I'd eaten the day before was converted to glycogen by my liver, but before being converted to fat, converted back again to glucose to power my mornings. I used to ride home and do an even longer paper round then (I had to call back to the shop for 1/2 the papers because they wouldn't fit in 2 satchels and trying to ride with more than 2 was a major ballache). Now, TBH, I could pretty much hide behind a lamp post, but my musculature was very well toned. I too used to play rugby for my school and also sprint 100 & 200m for a local club, plus I'd play football with mates for hours at a time, or go swimming, or our own sports = cross between hide & seek / tag on bikes but you captured people and they could be released by uncaptured players (releasio) and team tennis on quad skates, played on a length of street about 80m long and the subject of many disputes since there was no net, oh yeah and fighting - there was fuck all else to do!
I only found out about the glycogen to and fro because I applied to the army at 16 (I was still riding a lot) and rejected on medical grounds due to protein in my piss. Turns out it's a normal side effect of running your body at peak efficiency, but by the time this was understood, mates of mine who joined up a year ahead of me were home on leave and they had become racist, sexist, big mouthed pricks, so I decided against the military... nevertheless, even though I didn't keep up sports much past about 17 the fitness stayed with me into my late 20's.