I have made a pretty similar leap between junior and senior year. For me the trick was keeping really consistent mileage and having a few bread and butter workouts every few weeks that I could track my progress on. You should talk with your coach because different things work for everyone, but this got me from ~5:00 to low 4:30s currently. I was hitting 55mpw (pretty heavy duty for HS, talk with your coach!). I ran 16:45 in XC on a pretty tough course and was sick the week of our fastest meet of the year, but I estimate I had around 57s speed like you. Over the winter, I was completely in charge of my training, so I took a week off after State XC and then started up with the same mileage and consistent work in several pace ranges. I was doing a tempo/Vo2 and then a speed workout every week as well as a long run. My main workouts looked like:
4 mile tempo. 5:50 start of winter, 5:40 by track season
5-6x800 on equal rest (slightly faster than 32 pace). 2:32 average start of winter, 2:27 by the end
8x200, flat or hill, jog back (60s) rest. On track, 31 at the start of winter, 28-29 by the end.
These weren't the only workouts I did, but I would throw some of these around fairly often to make sure I was still moving forward in training. I also very often did 4-6 strides at the end of every easy run to work top speed that I lacked after doing slow twitch work for so long. A month before the season I did 10x400 75 sec rest averaging 68 (predicts 4:32 mile) and while I debuted the season at 4:51, I eventually worked my way down to that low 4:30 time. Before the end of the season, I think I will break 4:30. You may not have to resort to doing intense self-training on your own, but lacking a lot of teammates and a thorough support system, this is what I came up with. I'm sure many coaches on this forum will point to obvious flaws in my training but it worked through guts I guess. Some mistakes I made:
-Not doing much top-end work. I think I should have done some sessions like 5x60m top speed fly with 5 min rest which would have improved my 400 speed greatly (necessary)
-Once I got in-season and stopped training myself, I was hardly being told to do any speed work. Reintroducing some fast 200s has helped in the last couple weeks. Make sure you touch on some consequential speed work fairly often
-I was frequently running into recovery and overtraining/injury issues. You should be hydrating, stretching, and probably rolling yourself out.
-My training, though I updated my coach during the offseason, was not very coordinated with his philosophy, so once I got in season, he started training me completely differently and I lost a lot of shape right off the bat. Coordinate more with your coach and if you do train yourself, talk about how your workouts are going to change going into the season.
-No weightlifting. I think my 400 speed also didn't improve much because I didn't progressively overload the muscles with resistance training or specifically attack the 400 as much as I should have. Having made a lot of improvement in the 16 and 32, my 800 time hasn't budged much and the lack of 400 work (long rest and very fast intervals) is the culprit.
Here are some things I think I did well:
-I didn't peak too early. I tended to stay away from those really big race-specific sessions like 3xmile @ 3200 and it's led to me having a lot of room to grow during the season.
-I stayed religiously consistent. Fell asleep afterschool and it's 8 PM? I ran (including workouts). Emotionally bad day? Still ran. I can truthfully say that barring illness, I haven't missed a single session in a year and rarely ever had to cut mileage. This included family travels and doing a lot of treadmilling at the local rec center. If you're injured, ellipticals and stationary bikes are great non-impact cross training too.
-I was able to keep my cool. This being my last season (no college running most likely), things mattered a lot to me and that's the reason for this ridiculous journey. But still, I never mentally burned myself out or suffered major race anxiety. I think this has been very beneficial to still being in it and willing myself through tough workouts even today.
This is all pretty basic stuff and I screwed up in several areas, but that's life, especially life being young and learning. For being my first time really self-coaching, it has turned out alright. To answer your post directly, 6 days a week with good mileage (maybe a shakeout on the 7th day) for a year was the way I got here. While you're base building in early summer, it's good to get those 4 mile tempos but don't sweat much else as you get closer to competition. Best of luck!