eliminating fast food/soda from diet
don't get me wrong here, i think getting your baseline nutrients is really important, my thing here is that i never stopped eating junk, and drank a soda or two a day. in fact, it was probably good for me calorie-wise. eat your veggies, and a couple of cokes or burritos are not going to ruin your season.
stretching
especially the classic "stand in a circle and hold positions for 10 seconds" kind of static stretching. i feel it does nothing for you and is a waste of time. active rope stretching is useful, but still time consuming.
if i am going for a distance run, the first mile is the warmup. no stretching.
if i am doing track work or racing, i'll do a light jog as a warmup and some skipping drills. 100m to 10k, i have not done a stretch routine before a race since sophomore year of HS.
high mileage for high school runners
this one is more conceptual and no longer applies to me of course. there's no need to train like a 34 year old marathoner as a high schooler who is going to race 800m and 1600m on the track. yes, there is a 5k in cross country. that's as long as it's going to get. let it take a backseat. 4 years of high school is not enough time for meaningful aerobic development off of low intensity.
if you want to compete in HS, you run an intense 40ish mpw, race frequently, and blast 400 repeats just like everyone else. teenagers recover from that stuff.