In order to break 2:12 for the marathon your need to train in such a way as to have the Aerobic Power, Aerobic Resistance and Specific Endurance necessary to accomplish this goal.
Sub 2:12:00 would be 5:02 per mile or faster
Aerobic Power
2 benchmark workouts that would indicate your Aerobic Power is in good enough shape are:
- 6 x 1 mile @ 8% faster than goal marathon pace (GMP) with .25 mile jog recovery
- 6 mile Tempo Run at 4% faster than GMP
Aerobic Resistance
2 benchmark workouts that would indicate your Aerobic Resistance is in good enough shape are:
- 20 miles steady state at 5-6% slower than GMP
- 24 miles moderate at 10-12% slower than GMP
Specific Endurance
2 benchmark workouts that would indicate your Specific Endurance is in good enough shape are:
- 20 mile run with first 5 miles easy, the middle 10 miles at GMP or faster and the last 5 miles easy
- 10 mile Wave Tempo alternating miles between 3% slower than and 3% faster than GMP
Math:
5:02 per mile is 2:11:50 for the marathon
8% faster than 5:02 is 4:38 per mile
4% faster than 5:02 is 4:50 per mile
3% faster than 5:02 is 4:53 per mile
3% slower than 5:02 is 5:11 per mile
5% slower than 5:02 is 5:17 per mile
6% slower than 5:02 is 5:20 per mile
10% slower than 5:02 is 5:32 per mile
12% slower than 5:02 is 5:38 per mile
These benchmark workouts need to be able to be done in the normal course of training, meaning that they should be able to be done and recovered from with 2-3 normal easy days afterwards. You should not have to taper and race the workout in order to accomplish these.
Seek to be able to accomplish the Aerobic Power benchmark workouts first in your training cycle before focusing more significantly on Aerobic Resistance and Specific Endurance as race day approaches.
What other workouts you do in your training (yes you need to do other workouts) will depend on which of these areas you are weakest at and so what you need to work on most to get up to these requirements in these specific areas. My experience is that most runners from a shorter distance background can more easily reach the Aerobic Power requirements but struggle more reaching the Aerobic Resistance requirements and the longer of the longer (5E/10T/5E) Specific Endurance requirements.
This is where your coach comes in to know your weaknesses and strengths and design your training so as to be able to help you improve your fitness in such a way as to meet these requirements by goal race time.
The mileage needed to be able to meet these requirements will vary depending on the individual. Some can can meet these requirements on lower mileage and some need a higher volume to get there (specifically for the Aerobic Resistance). I have seen elite runners who required half the mileage volume of other elites to meet the requirements, so it is largely an individual question. The key though is to do what you need to do to be able to meet these requirements.