Nothing is unrealistic. I'm just trying to steer you towards something that might give you better mileage for what I assume your needs to be (of course I shouldn't assume these things, but at this point it just makes straightforward advice a lot simpler). The best all-around backpack for a novice backpacker is an internal frame pack, in my opinion. Though you could of course get a UL pack as your primary pack, I think you may feel constrained by it since as soon as you overload them, they tend to hurt your shoulders, whereas an internal frame pack, once overloaded, doesn't hurt your back directly--rather it just makes your legs do more work.
Before you buy an ultralight backpack, though, I recommend just weighing out your gear to make sure it really is as light and compact as you think. 15 pounds for 1-2 days, including food, water and stove fuel might be a little different from expected when you actually have it on the scale, and I am using "may" herevery loosely. It took me A LOT of practice packing a backpack to get a comfortable gear list weighing only 15 pounds (with consumables) and that included a 1 pound sleeping bag, 1.5 pound backpack, 12 ounce tarp, and an 8 ounce sleeping pad. It's easier than most people think to pack 40 pounds of gear and consumables into a backpack, and far harder to pack less. That's why we have websites like this, after all...
Packing strategies aside, the point I want to stress is that a sturdy internal frame pack going to be comfortable at pretty much all weights whereas ultralight packs are really only comfortable for less than 25 pounds-- less than that even if you don't have shoulders of steel. I never put more than 20 pounds in an ultralight backpack when I have the option of using an internal frame. So if this is to be a backpack for family excursions, carrying other peoples' gear, or a backpack for week-long trips, or a backpack for winter trips, I'd go with the internal frame as your do-it-all pack.
Then again, nothing is unrealistic and if you want to get into UL backpacking from the outset, I wish you well. ULA and Zpacks are my favorite UL backpack makers.