Most backpackers don't do 20-mile days. I think those that do are mostly through-hikers hiking the long trails like the PCT or Appalachian Trail, where they need to maximize their daily mileage in order to complete the trail in one season. And they don't start off doing 20-mile days, but build up to it. There are exceptions, of course. Some very fit hikers may go that far daily on an "ordinary" trip, but they're a small minority.

For perspective, I would imagine most people go 5-12 miles in a day on a typical trip. At least, that's true for me. A lot depends on the amount of elevation gain, and of course, pack weight.

I've never done a through-hike, but it seems that most of the people who do are able to build up to 20-mile days, occasionally 30, and very occasionally 40 miles. With very light loads.

In any case, it's not a contest or a race. Whatever pleases you is the right way.