Overdoing the mileage especially at the start can spoil the trip, but I don't agree with comments that often come up suggesting that riding all day doesn't allow doing anything else or enjoying the country traveled through. I'd like to point out that the countryside can be enjoyed from the saddle, pictures can be taken on stops and stops can be made specifically for the scenery.. You can talk to the local folks at the stops if you make the effort. You can eat the local cuisine.
I am often surprised when comparing notes with other riders in camp that some of the folks who supposedly are taking the time to "smell the roses" actually haven't really taken much advantage of their down time to do much mingling with local folks or doing much. On the other hand I have found it possible to meet and chat with people, eat local food, and enjoy the wildlife and scenery even on the very longest days. For example on my longest day (142 miles) I enjoyed beautiful scenery, saw a lot of wildlife, met and had conversations with a few folks (some in mixed English and Spanish with neither side having much grasp of the other's language), sat at communal table for breakfast with the locals in a local diner, had lunch in a place frequented by non English speaking folks and ordered from a menu that I couldn't really read, outran a huge thunderstorm, and at the end of the day I negotiated a really nice discount in a beautiful room that would normally not have been in my budget.
I am not saying that all folks who do short miles sit in camp and do nothing, but I have been surprised how little correlation there is between mileage and actually seeing the sights and enjoying the trip. You can do long miles and still see and do lot if you make an effort to do so.
I think one of the things that does make a big difference is that I am not inclined to sleep in or sit around in camp in the morning. If you roll out at 5 or 6 am it gives you a huge advantage over someone who rolls out at noon or even someone who rolls out at 8 or 9. You have a lot more time to ride, a lot more time to take breaks, a lot more time to talk to folks, and you can knock of earlier for the day if you don't want to knock out long miles.