4:54 sophomore 1600 to 4:35 Junior (HS)>?

40 is more than enough! Focus on good recovery and try adding in some basic speed when track season comes around.

Many HS runners enjoy a big improvement from their sophomore to junior years, and for others, not so much. You should take the old fashioned approach of training and racing, and seeing what happens.

You will most likely see about a 10 second improvement this year.

Talk to your coach about goals for each XC and track season. If you stay healthy and train intelligently with progressive increases in training load, then your goal is reasonable. Unfortunately, HS kids are still growing and that can cause setbacks (for good reasons).

35-40 miles a week will get it done for sure. I went from 4:47->4:30 in one year from simply running a bit more in the 50-55 range during cross and ran about 17:04 before I ran the 4:30 that outdoor. So not aerobically gifted in that sense. I did lots of jack daniels workouts and that got me pretty far, better pacing for races would have served me a bit better though. Just run and eat and sleep a lot and you will get there!

you are in the ballpark. I ran 5:07 as a freshman, 4:53 as a sophomore, and 4:39 as a junior. I averaged about 40 miles per week my junior year.

Generally runners see big improvements early on, then the size of these improvements decrease over time. Improving all the way down to 4:35 this year would be a very large improvement. Not totally unheard of, but possibly not realistic too. However, your mileage is pretty low, so there is a lot of room for growth there. Many high school juniors in the US are running 60mpw or more. If you truly want to improve your time a great deal, my recommendation to you would be to try to run doubles starting now ish. At first, just add a 2 or 2.5 mile morning run to your week a couple of days. Do that one or two weeks, then make it 3 or 4 days per week, then 5 days per week, then increase the distance to 3 miles. At your age, I would not suggest making the morning run more than 3 miles. Continue the rest of your training as normal.

If you're going to go this route, I cannot stress enough that adding mileage greatly increases the risk of injury, so you MUST do it slowly and gradually and give yourself a day or two off if you ever feel a pain coming on. Unless there's something anatomically wrong with you, you should be able to tolerate 50-60mpw as a high school junior though, AFTER SLOWLY BUILDING UP TO IT (over the course of a few months).

Finally, be sure to do lots of strides with plenty of rest, and try to never run too hard on threshold type runs. You can get a lot of really solid, beneficial training in even from "threshold" runs that are 5-10seconds/mile slower than true threshold pace. Several months of this type of training is far superior to running too fast on these types of runs.

Anyway, good luck. For reference, my progression starting in 7th grade looked roughly like this:

5:01
4:42
4:30
4:23
4:13
xxx (injured senior year)

10 seconds' improvement is a more sensible, attainable goal, 15s is usually the outward bound for most people. 20-30s is like you were out of shape before, maybe running 530 when you started, and getting fit gets you to 5. but as you get fitter and faster each second downward is tougher to get.

you want a goal you can get. and then you reset the goal a second time. that makes it a happy experience. you get the goal and reward. you chase it again. that's fun.

people setting massive goals are often miserable because they get 10-15 seconds' improvement they should be happy about, and d3 interest, but they are upset they had some idea in their head they'd drop 30 seconds in a season and go from JV runner to d1 recruit in a season.

and if you are the rare one who gets that done, doing 2 or 3 goals in a year at 10s chunks doesn't hurt you.