So my current fitness projects to 3h30 in the marathon but I want my next marathon to break 3 (in about 6 months).
I plan to run 70-80 mpw but I wonder if i'm wasting my time doing Daniels' Marathon paced workouts instead of working to get my threshold pace down even further.
There are some weeks, for example, where i'm not doing any threshold running at all; instead its just Marathon stuff.
Should I make the switches?
How much time have you been running ?
Why 70-80 miles ?
Your weight - height, age etc. more info about yourself, PB's, last half year mileage?
For sub 3 h marathon you should run about 1.20-1.24 HM and 37-38 min 10 km.
These 6 months should be structured into base-phase, pre. competition phase, competition phase (at least).
Also you should do different workouts during that half year.
First of all, write down in details about yourself.
Age- 26
Height- 5'8
Weight - 152lbs (slight muscular build)
Half PB (last year): 1h31
Last half year mileage: 20-30~ mpw with very sporadic running-- never ran anything faster than very easy pace ; just relaxed jogging throughout my summer and I made no fitness improvements. I lost a lot of my fitness that led me to my half PB and I'm starting a bit behind.
The 70-80 miles is because I have run that mileage during my half training program (10 weeks) and my legs are quite injury resistant.
I'm a disappointing starting point but the past 3 weeks I've already been building my base back up to 75~ mpw at around 9:20 min/mile pace
I don't understand why you run 70-80 miles at 9:20 for running only 1:30 ?
That is very slow pace and too much time on feet.
Start to run 40-45 miles 1-2 weeks at 8:30-8:45 pace with 10 miles LR and build up step by step.
Also you should do 1 tempo/cruise intervals + another session could be faster one.
You can run 100 miles at 10 min per mile and your result will be 3:15.
My point is train smart or try to understand how to train smart and what to do for running faster times.
In my opinion it's too early for you to run sub 3 h in 6 months.
Firstly try to run faster 10 k and HM.
You said "current fitness project to 3:30", but what is your most recent marathon? Is the planned (or should we say, "desired") sub 3h your first?
If this is your first, be aware Daniels' (and others') marathon projections are based on not "hitting the ceiling" (i.e. lactic acid accumulation and blow-up). They all assume the issue of "hitting the wall" (glycogen depletion bonk) has been sorted out. In practice, the latter is the most difficult for most people (there is nothing like in HM and below). They bonk even when running well below the threshold (MP or slower). Your training should be focused on overcoming this (and dial down the target pace accordingly). If you're projected to be able to do 3h30 but haven't done it, the next 6 months should just be to ensure you can do that.
Let me try to actually answer the question that you asked.
One of the things that Daniels took out of the 3rd edition was a table that calculates threshold pace for runs of longer than 20 minutes. a 3:30 marathoner has a VDOT of close to 45 and a marathon pace of 8:00. If you were to look at the second edition rather than the third, you would see that for a runner with a 45 VDOT, M pace and T pace for a one hour continuous run are almost identical.
This stems from the fact that lactate threshold is not a point of inflection, but rather a continuum. Waste products build up more slowly at this pace, but they build up more slowly. The point is to get a material accumulation of those waste products over time.
So yes, I would keep the long runs at M pace in. I would, however, avoid shorter runs done at M pace as they simply do not allow for a long enough accumulation of waste products to effectively effectuate the desired adaptation.