The ST being mostly flatter and emptier, I'd think many folks manage more like 80 miles per day there. As a 60 something non athlete I averaged about 80 miles per day there.
I'll also mention that The ST's biggest advantage is that you can do it in winter. The scenery was kind of dismal much of the way in my opinion. The food and people were interesting though.
Pete, do you think your dislike of the Southern Tier has anything to do with your mileage on it? If I'm not mistaken, you averaged more daily miles on that tour than on your other rides. You found food and people (experienced off the bike) interesting, but scenery (most of which you presumably saw on the bike) dismal. MIght you have enjoyed your trip more if you'd taken more time?