5k xc time not improving, despite the high mileage

That's not high mileage.

Run some threshold and tempo runs.

I agree on the run tempos part, but these are your first races, it's literally august still.

I doubt you popped a 17:30 first race last season unless you peaked early. You're in 17:50 shape atm and it's only going to drop. Go for 17:40 then 17:30 and then 17:20 then...

Climb the ladder one rung at a time, you're in decent shape for early season, build off that

That's not particularly high mileage, but look at it this way, ur 2 races into this season that has barely started, and ur already basically at the same fitness as where u were at the end of last season. Plus I'm assuming you haven't really done very many if any high intensity workouts yet which will definitely make you faster once they settle in. You're doing fine and don't worry about it, if anyone on this sh*tty forum says otherwise don't listen to them.

make sure to be doing strides, makes a surprising difference even during cross country

It's high mileage for him, and it isn't working.

The term threshold has been outdated since 1985, but the gist is correct: more jogging around like a snail doesn't help much.

Try 3:30 km reps with 400m walk, about 10-15/week, keep the mileage about where it is.

And don't bother racing hard until the 3:30 reps are noticeably easier.

I have been running for 25 years. I set my 5k PR of 14:20 while doing 35 MPW. I tried 50 MPW my junior and senior years of college but couldn't manage to break 14:40. I dropped back to 35 after college and hit 14:30 several times. High mileage isn't for everyone.

Your season just started. It's cross country. Courses are not exact and every course has different features. Grass tends to be thicker this time of year, among other things. You've got mileage in the bank. It will pay dividends are your season progresses. Be patient.

When you increase your mileage, there is a period of time you will actually slow down. Your body is trying to adapt. Once it adapts, you see the benefit. But initially, it can leave one feeling sluggish from the increased training load.

You also say you did no workouts until the week of your first race. You go all summer with no workouts, then hit yourself with them the week of your first race. Of course your legs are going to feel flat, when they haven't yet had time to adjust to the new training intensity (and volume).

If you keep up the higher mileage and workouts, you'll get faster. It's just going to take a few weeks.

Increase mileage > adjustment period > Increase intensity > adjustment period > taper > race.

You did this all quickly, crammed into a few weeks and right before your race, and instead of tapering the week before the race to rest your legs, you fatigued them with new workouts.

Not necessarily or world class runners would be doing 200 MPW.

I'm running similar times to you in the 17 zone. Last year I was running / jogging 70 miles per week at one point which didn't achieve much, then this summer I have mostly been doing 40 miles per week and have run 17 / 36 in 5 and 10k. At our level I think quality > quantity and recovery is crucial. No point doing tonnes of easy mileage if it's not making a difference in your results and you are tired. In fact there is a guy in my neighbourhood who is a level above me and has run 16 high / 35 for 5 and 10k and his highest mile week is 35. He'll do a 2 hour run at 7:30 and sometimes an 8 mile tempo, runs 4 days a week.

This. However, if you continue to feel sluggish, get your iron levels checked, particularly ferritin. Should be > 30 and probably > 50.

Higher mileage base building (by itself) didn’t make me faster. Instead, it allowed me to handle harder, and especially longer, speed work. When I focused on threshold training and VO2Max training, then I saw the speed gains. Be patient. It’s still August.

Also consider other factors. How are you sleeping? Eating? Recovering from workouts? If those areas are suboptimal, they can hinder your performance.

This is the important point. Try to think of the work you have put in up to this point of the summer as foundational i.e. required to increase intensity down the line to be able to run fast times in October and later.

how fast should I be running my workouts? My goal is to break 17 minutes, preferably 16:30s-16:40s. bump

I had a similar situation as you at some point in the recent past. I ran 2:08 800/4:47 mile with a 5k of 17:40 on around 30-40 MPW. My 5k PR is now 16:18. The only thing I did to improve the 5k time was increase aerobic miles. This is running at a brisk pace where breathing is still controlled. You should finish runs feeling good. Add in some tempo runs, which are a bit faster, and less total time, but breathing is still controlled. I think if you focus on increasing time running at or under paces where your breathing is still controlled i.e. not labored you will get under 17.