It depends mostly on how strong a hiker you are - and that depends heavily on age and condition, which we don't know. I can't do any more than 10 or 12 and reach camp by dark; my buddy can do 15-20 by 3pm. (I'm 61; he's 28. I'm an accountant; he's a former SEAL who is now a firefighter, and thru-hiked the AT a few years ago. See what I mean?)

One difference I notice between dayhiking and backpacking is that I tend to take more frequent breaks when backpacking. Part of it is that I'm carrying more weight. But a lot of it is familiarity: I dayhike locally, and end up revisiting places a lot - so there's no need to stop very long at any given place; I'll see it again soon. When I'm backpacking, chances are good that I won't be passing that way again, at least not for quite a while, so I tend to take my time and soak it all in.