How to develop XC specific fitness?

You need to slow down on your tempos, as right now you're basically racing them. Everyone is different, but generally 5:45 for threshold is the ballpark of a 4:30 miler. If you're slowing down noticeably or having to increase effort to maintain pace during a tempo, you're going too fast.

This alone won't make you a sub 17 XC runner, but it's a big step in starting to train properly rather than overtraining. Remember this: you can only do workouts appropriate for your current fitness level and make steps that way. Trying to train like a sub 17 guy when you're an 18 min guy doesn't work.

Hills. Run hills, do hill repeats, run more hills.

I agree, although 5:45 for threshold would be on the slower side for a well developed 4:30 miler. Could get down closer to 5:20-5:25 pace with that speed.

You didn't really do base workouts during your summer training OP. You did a lot of 5k specific work, which could lead you a bit burned out a some point since the workouts you're likely doing now will just be piling on more of the same. Endurance does seem to be an issue. 2 mile tempos are too short. Aim for at least 3-4 miles. Slow them down. Don't race them. Aim for consistent split and/or a slightly faster finish. XC racing can be a bit of a toss up. Some courses are short/long, fast/slow. Maybe you're ready for something close to 17-flat on the track. That might be worth something closer to 18-flat in the race you just ran 19:07 in. Going out too fast (5:20) results in a steeper drop off. So if you opened in 5:30-5:40 you might've run 18-mid but, instead, you faded to 19 & change. Learn how to race. In your next race let some people go. Don't sprint off the line & relax in the 1st half mile. Make your 2nd mile faster or the same as your 1st. Set small goals like that & better results will come. There's a learning curve here.

he’s a senior boy who started running this spring. not a well developed (aerobically) runner.

I don't know what a "rolled 19:07" means so take this for what it's worth. Workout times are not good predictors of racing performances. Your expectation of sub 17 was probably based on those workouts. I don't know what sort of surface you did those workouts on but if was a track or road or a smooth trail you probably weren't fully prepared for the terrain you raced over so those workout times become even less predictive. Many good runners do not turn in cross country performances that are on a par with their track times and some good cross country runners are far less impressive with their track performances relative to their cross country ones. Finally, even if you are in shape for something like 16:59 now (which I kind of doubt) your first mile was much too fast and cost you a good bit of time, probably not two minutes, but a noticeable amount.

Now you have an actual racing performance and should adjust your expectations and come up with some reasonable goals both in terms of times and places,for your season. That's the sort of thing you should do with your coach. One day you might be in sub 17 shape but I expect you're a ways from it now.

I did. The post I responded to said 5:45 is the ballpark threshold for a 4:30 miler. OP hasn't run a 4:30 & their threshold isn't anywhere near 5:45. Simply pointing out a 4:30 miler can have a threshold a good bit faster than 5:45.

OP should focus on developing some strength & learning how to pace in their races.

You need to shorten your rest on workouts You're not getting the full effect of your training by taking so much rest.

Any workout at threshold or slower should be doable off no more than 1:30 rest - and ideally closer to 1:00 - no matter the distance of the reps.

Even for harder paces, rest probably shouldn't exceed 2:00.

If you have to slow down the paces to make your workouts achievable with this rest, then slow down the paces.

But since you're a HS athlete this kind of stuff is up to your coach. I would talk to him about it and see if you can do shorter rest on workouts. But at the end of the day you're likely constrained by whatever he wants/tells you, and if he doesn't budge then you will likely remain rolled.

More rest means running faster, so the net effect is the same.

Lydiard's guys had the idea of taking short rests with long reps and long rests with short reps. The idea was that longer reps was a session more aimed at developing strength and endurance and short reps were usually meant to develop speed. If you were trying to do the latter you wanted to run each rep as fast as possible so your muscles adapt to the kind of movement they need to do to run fast. If you're taking short rests as the workout goes on fatigue is going to make it harder for you to do that.

I would not have much confidence in you coaching any one at all if you believe that's true. Just a grossly oversimplified statement.

10 x 1K. Athlete A does it at threshold, say 3:15 per K with 1:00 rest. Athlete B does it at 3:05 with 2:00 rest. Did these athletes do the same workout??

Absolutely not, and you're a hack if you say otherwise.