There is no generic answer to this most-frequently asked question for Alaska backpacking trips. Obviously your fitness, your pack weight, your group, etc, all heavily impact the distance you’ll cover each day. More than that, the terrain itself will determine how far and how fast you travel.
Not just the gradient and uphill/downhill stuff. Those things clearly are important. However, here in Alaska, the most common determinant, and most profound one, is the terrain itself. The footing. What are you walking over? What are you walking THROUGH? Heavy, dense alder will slow you down way, way way more than you imagine. Add thickets of Devil’s Club inside that and you’ll be moving very slowly. You might make 3/4mile an hour. Maybe. Even on flat terrain.
Another terrain challenge we see a lot is rocks. lots of rocks. Glacial moraine can be super slow going. Huge boulder fields can cause some people endless balance concerns, and the result is a slow progress forward. Talus and scree slopes can do that same.
Not all people experience this stuff the same way. Some people move through alder, or over rocks, much more fluidly and comfortable than others. My nemesis is mud. LOL There are a few trips where I prefer to add a good bit of distance to my route and avoid a swampy wet sloppy mess underfoot. Other folks don’t mind it at all.
The point of this post is that you shouldn’t concern yourself with mileage. As clearly as I can state, it’s meaningless. Beyond meaningless. The mileage of an off-trail route tells you NOTHING about how challenging a trip might be, or if that trek is a good fit for you. Nothing at all.
In short, you might make 10 miles today, over good terrain, with no real elevation loss or gain. Nice walking. Tomorrow you might make 2 or 3 miles because you spent a few hours crossing a boulder field or a river or picking your way through alder thickets or sidehilling a talus field. And that day may be the most difficult hiking day you’ve ever had in your life. So don’t fall for the idea of mileage being some useful metric for your trip. It really isn’t. At all. Pay it no heed.
Don’t bother pondering it. Instead, practice carrying a 35lb pack over a boulder field for a couple hours. You’ll be glad you did.