1:55 may be a tall order, but it won’t be impossible.
I run 1:54 with 51.2 400m speed and 4:20 mile endurance. Where is your mile/1600 at?
You’ll need to do more lifting and general strength work as well. I’ve found better muscular strength not only helps with acceleration and top end speed, but also 400/800 type speed endurance. If you don’t have a lifting background, do some lighter lifts like goblet squats, kettlebell swings, weighted lunges, running arms etc. Youll still develop speed and power from doing this.
You’ll probably have a hard time doing anything more than 35-40 mpw cause 3 months just isn’t enough time to rebuild mileage. Structure for 3 quality sessions per week. I’d recommend one speed endurance session, one Vo2Max session, and one threshold/tempo session. This is what a training week could look like for you.
Monday: 5-6 miles easy
Tuesday: 1 mile up 2-3x100-200-300-200-100 nearly all out for each rep, take all the rest you need, 1 mile down
Wednesday: 5-6 miles easy
Thursday: 1-2 easy 3-4 tempo (feel free to do a consistent effort or k/mile reps) 1-2 easy
Friday: 5-6 miles easy
Saturday: 3-4x(400-200-200) mile pace or faster for the 400, 800 pace or faster for the 200s, 2 minutes between reps, full rest between sets.
Sunday: 3-4 easy or rest