I take a different approach on longer trips. Prior to the trip, I get a rough "mileage budget". For example, perhaps I'm going 3000 miles and ask is seven weeks ok. That is ~60 miles/day including my rest days. Based on my past trips, terrain and other things I might want to do off the bike - decide if that is OK or not. When I start the trip, I keep rough track of how I'm doing against my "budget". If I am "ahead", I might more easily take a rest day with inclement weather than if I am "behind". I might take alternate routes and adjust as I go. Very little of that has to do with my average speed because I'm not maxed out riding all the daylight hours. So if one day my average is lower - I just spend some more time in the saddle compared to another day where the average might be higher.
However, I don't pay much attention overall to average speed - either while on tour or even prior to the tour when planning the trip.
I was just thinking about this today and thought pretty much the same thing. Basing more on mpd than mph is a better way to plan, I think. Since I have to call myself a beginner, I'm looking at my mileage for each of the last three months and looking at my progression that way. Now if I can stay on track and still do my otr trucking job......