Recently discovered this thread so thought I might share my achievements.
Currently I am a Junior in high school, in a nice suburban town in Massachusetts (4th healthiest state I think?). I'm relatively short for a guy, and incredibly skinny. I'm 5'7" and 106 pounds. I have no idea what my %Body fat is but I can assure you it's incredibly low.
I've been doing endurance training ever since the end of elementary school in the form of swimming. I was never any good at it, considering I was tiny, and didn't have anything close to a growth spurt until 8th grade. However it did prepare me for other endurance training, and developed my body early on, which I am grateful for. I do the occasional swim now and then but it can't be considered part of my regime. Through the fall and winter of my freshman year, I did run a little, like an average of 2 miles a day, which was quite a feat for a freshman not involved in any particular sport at that time. In the spring, I took up track, which was easily one of the turning points in my life. I started training for distance, and found that with my background it was incredibly easy, so I just sort of skipped the beginner level of running. My first official mile time was 6:13, and by the end of Spring track, it was 5:45. I was pretty proud of my self. I continued to run over the summer notching 300 miles in a hundred days. Sophomore year I took up cross country. I sucked. I suddenly found myself doing 2 more miles on an average day than before, practices were longer, the weather was brutal, but I pulled through. My first 5k was 22:48. By the end of that season I had broken 20 and hit mid 19's. The longest run I did that season was 10 miles. I did 7 or 8 about once a week. I also continued with the acceleration runs, and found myself doing a highly effective routine involving my core which I'll get into later. Spring track my sophomore year, I almost broke 5 with a 5:02 mile. I was disappointed but still made districts where 5:03 was the qualifying time. I think I came in like third to last. Now here is where the real stuff starts. This summer I clocked 560 miles, which is 5.6 miles a day on average. 70 of those were done in one week, where I spent in Colorado for a cross country camp. The high altitude training does help, so if you're looking for a way to get some endurance in, and enjoy yourself at the same time, then I highly suggest hiking mountains, or mountain biking, or simply visiting a high altitude area and get some miles in. I have had experience with this since I was a little, considering I'm from Armenia where it's very mountainous. The village my relatives live in is about 6000 ft above sea level. This cross country season isn't done yet but so far my fastest time is 18:11 on our home course which is more than a minute improvement from last year. I continue to improve as well.
So that's my story, and I guess I'll give some tips.
Tips: Obviously running is a great thing to do if you're trying to lose weight, building endurance, or simply enjoying nature. DO NOT RUN ON A TREADMILL. However tempting it is to stay in your cozy basement instead of running in the cold, running on a treadmill is much more tiring and boring. If you're planning on running more than 2 miles, do it outside. Running in packs or with a friend is much easier and helps motivate oneself. There's more energy when running in a pack, and it's much easier to keep a certain pace and run with it. I am speaking with experience. If I'm running by myself 7 miles is the most I can do, but when running with a pack at a good pace, I could keep it up for a solid 15 miles, which is the longest I've run so far.
Body Weight Exercises: If you're looking for a solid core workout then I recommend the following. 90 seconds front plank, 75 seconds side plank (each side), 20 abductions (each side), 20 adductions (each side), 20 push ups, 20 crunches, and then as many pull-ups as you can do to top it off. If you're feeling good do two sets of all of them. You don't necessarily need to start off that long with the planks. It's something you build up to.
Hope I helped!