Volume of training can be a huge component to endurance independent of pace for much of that volume. Nobody is going to run world-class marathons for men doing less than 100-130 mpw, but the PATTERN WITHIN that volume is what will be important on "race day."
Here is a very simple formula for predicting one's best possible marathon time from your training preparation:
1. Total volume must be above 80 miles/week, ideally closer to 120 mpw.
2. At least 4 really long training runs of 40-50km in the 3 months before race day (with negative splits approaching race pace at the last 10km-20km of those long runs.
3. ADD UP YOUR 26.2 FASTEST TRAINING MILES IN EACH OFTHE 2-3 WEEKS BEFORE YOUR TAPER FOR EACH WEEK:
Example: You did 6 x 1 mile in all at 4:30 per mile = 27 minutes
You did a 10 mile tempo run in 50 minutes
You did another 6 mile tempo effort in 30 minutes
At this point you have 22 "fast supportive miles" so you need 4.2 more miles for your weekly "FAST TOTAL"
So your next 4.2 miles quick miles are finished in 25 minutes
Best Marathon Outcome Prediction: 27 + 50+ 30+ 25 = 132 minutes or 2:12 for the marathon
Your second week might have totaled 134 minutes for the fastest 26.2, so 2:14 for the marathon outcome
BEST CASE Prediction..2:13 for the 42km outcome
Essentially no BODY can do on a single day, single "workout/race" what you have not been able to do over a distributed set of workouts in a single week. Your cumulative "best week" outcome is the upper limit of what you can ask for on a single day!