Theoretical question. I do 40 miles per week. Just typical stuff - easy runs, intervals, long run and a tempo run (3-5 miles at 6:30). Ran 36:50 for 10k a couple of weeks ago.
What would happen if I did 40 miles at 6:30, nothing faster or slower?
You eventually would get hurt or be unable to hit pace from not enough recovery. If you manage to not get hurt and still hit pace you would likely just race poorly.
I did this in high school. Every day was 8-10 miles at 5:40-5:55 pace. My mentality was 'I have to work hard!'... I didn't know any better. I ran 9:12 3200 first meet of the year and 9:05 last meet of the year. You can run fast off that type of training, but you plateau, you don't reach a true peak. And I had pretty much no kick since I was just used to grinding all the time, so if anyone could sit on me long enough they'd out kick me.
You eventually would get hurt or be unable to hit pace from not enough recovery. If you manage to not get hurt and still hit pace you would likely just race poorly.
You don't understand tempo runs. There is no such thing as a pace to "hit."
i'm not surprised if there are so many definitions about "tempo runs" like shoe brands. anyway, if a moderate run is a 4 or 5/10, then the (lightweight) tempo run starts from 5.5/10, depending on the duration. it's "comfortably hard" and you can run a few per week. the result would be a solid distance runner.
and of course, there is also the "hard tempo" (8/10) with an ambitious given pace. this is a real workout (like the legendary "hard 10-miler"). some try to run pretty close to race pace (8,5-9/10) to test themselves or to shape up prior an important race. and you will be glad when a "hard tempo" is finished. we all know that nobody could do that for a few weeks.
Theoretical question. I do 40 miles per week. Just typical stuff - easy runs, intervals, long run and a tempo run (3-5 miles at 6:30). Ran 36:50 for 10k a couple of weeks ago.
What would happen if I did 40 miles at 6:30, nothing faster or slower?
If you can run a 36:50 10K, then 6:30/mi is about your estimated marathon pace. (If your "real" 10K race pace is faster, then 6:30 will be proportionally slower than your marathon pace).
So the theoretical question becomes, what improvements can 40mpw of training at marathon pace, with nothing faster or slower, bring?
One thing you don't mention is the range of training distances in a single day. Are some sessions shorter, e.g. 30 minutes, and others longer, e.g. 90 minutes, or are they all the same, like 1 hour a day, 7 days a week? Also, is the terrain flat or undulating? Changing these will provide a broader range of stimulation.
At a simple level, I like to think of training with three variables: endurance, stamina, and speed.
If you follow Jack Daniels, marathon pace is in the middle of a no man's land where it is not very effective at improving either endurance or speed.
Assuming 40 miles per week is not enough volume to get injured, you will become very efficient at marathon pace, but unless some of the runs are longer than 90 minutes, you won't have developed the endurance to run at that pace for much longer.
You might see some marginal improvements at 10K, and maybe even run a sub-3 hour marathon, but at some point you will stagnate, and you won't get the "bang for buck", that a variety of paces, distances, and terrain could bring.
You would get incredibly efficient at burning fat as a fuel source and likely get much faster if aerobic base is your true limiter and if that is actually your correct tempo pace and you're staying under 2.0 mmol on these runs.
Possibly repeatable but your intensity control would have to be near flawless.