I've heard the 25% rule before, but under that rule I shouldn't be running a 20 miler until I can handle 80 milesweek (I know its a rule of thumb, but still).
Right, that seems sensible. It is contextual, though, and as with about any given guideline, it depends. 25% could even be too much or it could be too little. Someone running 30 mpw might have no problem with doing 8-10 miles regularly. Brian Sell does not do at all close to 25% of his 120-160 mpw in a single run (and rightfully so), he tends to do quite a bit less. Yet in the Hansons plans for runners who only want to do 50-70 mpw, that is about what it comes out to. The longest run is 16 miles. As with any other workout in any training plan, how it fits into the context of the larger picture is as important as the details of the run itself.
With less than 3 months until a marathon and just now getting to 50 milesweek (and frankly I probably went from 40 to 50 faster than might have been prudent despite how easy it has seemed), I certainly don't have time or the experience to get anywhere near 80 (I've spent time in the 50s before and even done a few 60s, but never more than that). Obviously, I'm going to have to make do with the situation at hand, though it isn't ideal.
Then if you claim that you cannot do 70+ mpw then I claim that you should not be forcing 20-mile runs into such a low-volume routine. Being able to do 20-mile runs is not the same as needing to do 20-mile runs. Good marathoners likely could do 20-mile runs every single week, but then they also run 80+ mpw pretty much every single week. I am not sure where it exactly comes from, but it is a problematic notion to simply look at raw components of a given training routine and the pluck them out singly and try to ramrod them into a wholly separate and different routine. At any rate, consistency is the primary key of training for distance running and one will be more fit by spreading the 50-60 over seven days than dumping 20 of it into one day (or 23-28 into two days) and doing 30-40 in five days (or 22-37 in four). If one gets so run-down due to overloading one day or one weekend that a day off is necessitated then one would be better off backing off on that day or weekend and still getting out there every day that health will permit. One should be able to put sufficient strain on one's legs over the course of seven days, not just one or two.