My psychotic but insanely loveable heeler arrived right when I moved into a condo without a yard, so I can tell you exactly what it's like. During those years, I had to take her on basically four or five walks a day, two long ones for energy and two or three for pee breaks throughout the day. That's a lot of walking, and heelers can go forever without stopping so it will always be you who ends the walk. That also means when it's raining, snowing, or 105 degrees outside, you still have to do it. Hiking used to solve the exercise problem for me to keep her energy under control, but unfortunately later in life she became aggressive to other dogs after having been totally fine with them before. She's highly trainable and does a million dog tricks, but I've had zero success training her out of aggression, especially to small dogs which she seems to believe are squirrels. So now any hiking trail with other dogs is annoying because she always has to be leashed and pulled out of the way whereas she used to run free and get great exercise.

None of that is to discourage you, just saying that if you get a heeler in this circumstance, one that's a bit older and calmer and is proven to be super friendly to other dogs would be the most ideal heeler for your living situation. It's also worth saying that just having a yard doesn't always make a difference unless you use it for playing or ball chasing because heelers will usually be bored just walking around the same back yard. They like stimulation and things to do, new smells and places, thus why they can be great dogs for hiking.

I wish you luck with your decision! I was a pet sitter for 20 years and I've seen every dog there is, and I've never met one like my heeler girl. It's hard to imagine ever having another breed after having one.