For me a good mileage day is one where I can turn around and do the same distance the next day, the day after that and the day after that. Pushing extra hard for two days in a row but then being mentally/physically wiped enough that I then take a half day off isn't as much fun.
With that in mind, a pace where I'm doing 100km a day is a good sustainable pace and generally after getting in my 100km I think I've done a good day. I've done triple that in a day and if necessary after riding the 100km I can typically go another 50km if necessary to find a place to stay for the evening - but repeatedly doing 150km starts to wear me down after a week or so.
Yeah, that pretty well nails it, if you ask me. I always shoot for 75 to 100 miles and, after a few days of that, always wonder "why did I do that?." It's not that I can't do it; but it gets to be more like work than like vacation.
Old Town, check out the "long distance cycling" forum as well. It's... uh... well, it's just a different sport, that's all.