I'm doing a similar return to serious running -same mile PR, slightly faster PRs at longer distances, slightly younger (28), and less removed from serious training (~5-year period of no consistent high volume). It's been tough thus far.
Given the available context of your situation, sub-4:00 seems delusional to me, but a goal can be delusional and still be a good goal depending on your mindset and life circumstances. It can be good to have a crazy thing that gets us excited and out the door every day, and with the right mindset will be a fantastic project even if the odds of achieving the crazy thing are quite low. It's bad if we over-invest our time and energy into the crazy thing at the expense of other aspects of our lives.
Some important questions:
- Were there some tangible aspects of training/performance that you truly left on the table in college? Do you have a clear way to improve on these aspects moving forward?
- Do you have a clear idea of what training strategies work for your particular talents? You're at an age where there isn't any time for failed experiments before you age to far beyond your prime.
- Do you have (or can create for yourself) a sufficiently training-friendly lifestyle to properly execute and adapt to the level of training you would need to do?
- Do you have a lot of disposable income for gear, treatment, races, etc?
- Can you eat and sleep like a professional athlete? (and actually enjoy that life)
Some of my recent experiences/random thoughts:
- It's easy to get back to solid casual training, but absurdly difficult to get back to REAL high-level training. It took me over a year before I was durable enough to stack good training weeks without always dealing with some sort of small injury.
- It's a lot of fun to write my own training and approach a long-term plan, but also tough to balance both the patience and strong self-belief required. Feels weird to believe that I can be better than I ever was when I just ran a 5000m time where I'd get lapped by the college version of myself.
- Getting frequency of training was key for me -things finally started to click when I scaled back to smaller runs and workouts than I would normally do and added a lot of cross-training doubles.
- The new shoe tech is beyond awesome, nothing much more to say about that.
- Like you, I kept a decent level of speed, but my body's ability to do and recover from the high-intensity training to improve or extend that top-end speed is much poorer than it was when I was 16-22.
- The way training fits into life is fundamentally different now. Even without a family or a demanding job, it's just consistently less convenient to find space for running compared to my college days.
- In college, I didn't understand how much of what I thought was all my own internal motivation was outsourced to my teammates.
Training-wise, if I was in your shoes right I'd first focus mostly on pure speed stuff and pure aerobic stuff, including a lot of cross-training. At your age, speed is going to get harder to earn every year and it's much easier to maintain it than to gain it. Aerobically you are way behind on development and volume, but with cross-training you can accelerate that somewhat while mitigating risk. Outside of the sprint work, I'd keep most of the running fairly easy at first, maybe some occasional chill threshold intervals but not much/any hard "lactic" stuff.
Once you've established a quality and volume of movement over the ~3-6 months I'd then add in a lot of threshold interval work and medium-length hill repeats (30-90s). Do this for another ~3-6 months and now you're ready to start your first cycle of specific mile training.