Hey, didn’t see this post until now, I started my highschool XC career a good 30 lbs overweight for my height, I could run under 7:00 for a mile but I’m a guy and also had SOME fitness from playing middle linebacker/ tight end before joining, but I couldn’t crack 30 in a 5k and often had to walk on our team distance runs and couldn’t finish workouts.
I personally found success doing a decent volume of short intervals, as I could run relatively fast 100s and 200s, so I’d go and run my 16s/17s and 35s/36s with usually a 200-300m walk rest, that helped me lose the weight and started building my mechanics and let me hold on for our team hill reps, longer track reps, and eventually I found myself holding with the guys on easy runs, even though it wasn’t easy for me. On the tempo runs I’d run with the girls and I slowly just kept moving up as my fitness increased and my weight decreased. A year later I was a 19:30 guy and then a 2:25 5:20 guy for track. Still not great but nearing that upper JV zone where I didn’t feel like the team slow kid anymore. Flash forward a few years and now I’ve ran in D1 and some times I’m very proud of.
You just have to be patient, it really takes 2-3 years to really get rolling with running if you didn’t come into it being a lean kid who played sports that develop endurance throughout your childhood. I’m not saying everyone has the genetics to go and run D1 just by training hard, but most kids can be a good contributor at the highschool level with patience and smart training.