Not many people can "easily" average 25mph for over an hour. A 40K TT (24.9 miles) in an hour is a pretty good benchmark.

Note that because power requirements increase with the cube of speed, it takes nearly 75% more power to go from 16 to 20 mph, only 25% faster. You can play around with the exact numbers at various sites.

Standard advice is to not worry about your average speed, just keep riding, and eventually when you plateau, you can think about more structured "training" if that interests you.