Ok, I'm assuming you're not a troll and have genuinely run 4:10 and sub-2 off a casual 15mpw.
Increase your mileage. Aim for somewhere between 50 and 70 per week. Get there slowly - add 5 to 10 miles a week, there isn't a formula. The general advice is increase mileage by 10% per week but just go by how you feel - think you can add in 10 miles this week? Do it. Not feeling it? Wait until next week.
Do a long run of 12 miles every week (start at 8, work up). 10 mile easy run twice a week, fairly slow pace. 32 miles already.
Two 4 mile tempo runs (hitting a faster pace, let's say 5:20 to 5:45). Up to 40 miles.
Couple of 5 or 6 mile recovery runs, up to 52 miles.
If you want to keep up your long/short interval session pattern, you'll total maybe 6 or 7 miles between the two of them. 59 miles.
Couple of 5 mile base runs, hit your "natural" pace. 69 miles.
Add in a hill session.
70+ miles a week, suggested breakup is off the top of my head, just a general model. It really depends on scheduling vis a vis mileage dedicated to recover etc. Feel free to hit 80+ miles once you've built it up.
As for predictions? I would say assuming you're not a troll, comfortably sub 4. Whether or not you can go sub 3:50 depends on how seriously you take your quality mileage and to an extent your natural speed. Sub-2 800 if genuine is positive.