CD, I think what sub17 is referring to is about training to get the best out of oneself. No one is going to run their best possible marathon on 40mpw, it is not going to happen.
I'd disagree with your assertion about 'no-one' but that's another discussion. Sub-17 said "The long run should not be more than 33% of your weekly mileage", which is just wrong. Training plans should be written around the key sessions in any given week - mileage (and the proportion made up of the long run) should be incidental to that. Some plans have more sessions and therefore higher mileage, that's all. If the key sessions that get the best out of you happen to give a lower mileage, then so be it. Plodding out an extra 5 miles a day to add 30 miles to your week won't make you a better marathoner.
In regards to mileage and long runs. The long run should not be more than 33% of your weekly mileage. For 17 to 20 mile long runs you would be running 50 to 60 miles per week. More conservative plans have the long run at 25% of weekly mileage. To run 17 to 20 mile long runs you must be running 70 to 80 miles per week.
When you say 'must' you mean if you want to run to those conservative plans you're referring to. However it's really not necessary to be that conservative, plenty complete marathons at the front, mid and back of the pack on far fewer weekly miles than that.
Some good suggestions but bear in mind if you find routes online that they may be summer hiking routes and not necessarily available during ski season.
Out & back routes, along Lit's suggestion, are usually foolproof if a bit dull. Any attempt to work out anything else ("just turn left 3 times and I'll be back where I started") is guaranteed to end in failure. One guy on the sub 3 thread managed to turn a 3 miler into 20 using that method!