Personally, I think you’d be short changing yourself by hitting the treadmill since I can tell you from personal experience that running on the treadmill definitely is not the same as running on solid ground. In fact, I would be greatly concerned about pounding out 20 miles on the treadmill if I had not been doing treadmill runs already. I had to hit the treadmill a couple of weeks ago due to dangerous weather and an easy 8 mile run, the shortest run I bother doing for the most part these days, left me hurting so badly I actually contemplated whether I would have been better off not running at all than subjecting my legs to even the easiest of easy runs on a treadmill. I wouldn’t want to know what a long run on the treadmill would do to my legs, I probably would be out of commission for a week.
As for the pace and the water breaks, don’t worry. Personally, my paces always take a dive every summer. In the fall, they have never failed to come back and I don’t think my fitness has suffered from the slower pace in more difficult conditions. As far as the water breaks go, it is probably best to remember that Lydiard’s statement about 12 seconds was most likely stated as a relative condition. Relatively speaking, you lose some benefit of the run when stopping for more than 12 seconds. The question is how much are you really losing? I doubt it’s enough to really worry about. In fact, I’m so worried about it that I have never been concerned about stopping for even a few minutes in middle of a long run for a water break, pit stop, train, etc. While I try to limit the stops, I’m not afraid to stop if necessary. Of course, if you are concerned with this, another option is to only stop long enough to pick up a water or sports drink bottle and drink while on the run.