I find that no matter what shape I think I'm in, it's only a few days in that I am able to sustain the 50-10 or 55-10 ratio of hiking to break. It usually takes building up from around 30-15 or so and then eventually I take breaks just because I scheduled it that way (or there is something to look at, so we plan to hike until we get to a worthwhile point).

On weekend trips it's always a little bit of guess-work to figure out how many miles I think I will do.