I have some experience with this, so here's my longwinded thinking (for what it's worth)
Given that you are mainly doing 5K/10K mileage, and your 400 PR is 61 at 21yo, I'd guess that you can most likely improve your 400m time by adding some workouts to help you transition to running at speed.
Your first goal, I'm guessing, is obviously to break 60. You'll probably get a lot of suggestions on the boards here re top-end speed development, but that would require reducing your mileage -- and it sounds like that's not what you want to do. It may also be a shock to your legs to suddenly introduce fast stuff like flying 30/60's or flat-out 150m repeats. So in your case, I wouldn't suggest that sort of approach.
Here's a way to transition your legs to faster speeds, while keeping your mileage up and still aiming to run between, say, 56-60 sec:
6 weeks, 2 "transitional speed sessions" /week.
1st weekly session: In-and-out 100s, slow-jog the curve, fast on the straight. 1600m total. After warming up, do this workout fresh, before any mileage (if you are also doing an easy run). 1st 2 weeks, run the fast 100's @ 16-17 sec, next 2 weeks @ 15-16 sec, 3rd 2 weeks @ 14-15 sec.
2nd weekly session: Also do this workout fresh. 4 x 300m with 5 min standing/walking rest. Take the full recovery. Workout effort should be 90-95%. It's ok if your first session is 48 sec repeats or whatever, you just aren't used to running faster yet. Just let your legs adjust and your 300m sessions will improve. Try to get them down to 42-43 sec if possible.
If you're looking to do this over more like 12 weeks, I'd suggest the first 6 weeks to do 2 hill sessions per week: 12 x 200m hills at mile effort with 3 min walk-back recovery, and 6 x 400m hills at 3K effort with 3 min jog-back (then standing rest) recovery. I think you'd be more likely to get better results with this 12-week approach, because going into the (above) 2nd 6 weeks, you'd probably be stronger/faster.
If you want to add 6 more weeks, then after both of the above phases, I'd suggest the following:
Session 1: 4 x 150m fast, full 5 min rec. 95-99% effort.
Session 2: 600m, 400m, 200m, full 8 min rec. 90-95% effort on the first 2 repeats, 98-100% on the 200m (understanding that you will be tired going into it).
....
Any way you go, after completing 1, 2, or all 3 phases above, the next following week is your TT week. I'd suggest laying off all distance runs for that week, and running a super 400m PR with something like this:
Sun - 4 x 200m @ goal TT pace, full 5 min rec
Mon - off
Tue - 3 x 200m ...
Wed - off
Thu - 2 x 200m ...
Fri - off
Sat - 400m TT
Doing these suggested workouts means that you'd probably have to stop your 5K/10K hard sessions while doing the above. Just easy runs, and maybe one long run or tempo run per week.
If you do all 18 weeks (plus the 'rest/sharpening' week), you may surprise yourself. But it depends on how many weeks you want to dedicate to this experiment ...