Would <3:30 be possible?
Short answer: just about - if it went perfectly. But it won't.
Long answer: unlikely to hit that pace. 4:30 would be very doable though if you completed even 16 weeks of a simple programme, and if you got a couple of half marathons in there soon, you could get a sense of how close a sub4 was in a year's time.
You don't seem to be a newb in terms of time commitment (40+mpw at 10:30 easy pace is prob 6-7 hours of running weekly), rather, inexperienced in what to do with it (efficiency). I assume (and hope) your 40+mpw is not all at 10:30 pace? Also if you are regularly running 12-milers, you should be in a position to find out how fast you could cover 13.1 in. This in particular is worth finding out, if you are seriously thinking about making plans for a marathon. More important than the 5k.
Advice: (I'm assuming you're serious, so have answered at length).
12-month training cycles are too long to visualise, use smaller cycles.
Given mpw and current LR of 12 miles, get a time for 13.1 next couple of weeks, rather than a 5k.
Then, spend 8-12 weeks preparing for a half-marathon PB effort (should end up about the start of 2022, maybe the holiday week).
The VDOT of even this second half marathon will only tell you what your marathon could be, if you trained for it as well you did for the 13.1 at that point in time. Aka, the roughest of rough targets.
Take 1 week rest then and map out a 9-month block. Book the race. Address aches/tightness, preferred footwear/clothing/nutrition pattern etc that the half exposed.
40-45mpw is an ok launchpad for a first marathon block. Increase the long aerobic run up to 20/22 miles in due course, which will naturally increase the overall mpw volume. Use all of the time at your disposal to increase mileage realistically. Use runs for shorter, faster work through the week - on track (800s), on roads (fartlek) or even just trying to negative split 3-4 milers. Midweek volume may not increase, so long as it is used for intensity.
A taper of about 2-3 weeks will be plenty, some prefer just the 2 but 3 allow it to be not a complete drop-off. I suggest 3 weeks from longest run to race day.
Pace conservatively (i.e. go out in no faster than 2 hours for halfway), let the second half be whatever you can do. The theory should allow for a sub-4, in reality, you take what you get.