"knowing how I handle miles on different types of terrain would be helpful when planning longer trips."
There are a number of things to consider when trying to gauge your "rate of travel" over various types of terrain. The short answer is "experience is the best guide".
Miles alone don't tell the whole story. You have to factor in elevation, temperatures, pack weight, elevation gain, AND loss (for many, 8 miles of steep down hill would be crippling.)
For instance, I did a part of the CDT last year that has had No maintenance over the last 5 to 10 years. Lots of down timber, creek-like trails, stock-trough trails, no signage. But a piece of the Colorado Trail was like a freeway. On the first I was pushing to get 1.5 mph. On the second 2.5 was easy. And the guidebooks read the same.
The best you can do is learn from your training hikes and then extrapolate from the guide books, trail notes, and hiker reports.
And after three or four 'long' hikes, you'll know what to believe and what to expect.