I've placed many, many adult GSDs successfully in pet homes very similar to yours:
first time owners, no training background with no interest in IPO (but open to competing a basic obedience class), not long-distance marathoners but the dog gets walked daily and goes on weekend adventures, etc. As long as the rescue is being thoughtful about putting a relatively easy-going, low-drive dog in the home, these placement can be fantastic for adult dogs. We tend to have dogs we classify as "experienced owner only" and "great for a first-time owner." Lean on the rescue to help you identify the right dog, and wait until they have that dog.
Realize that many of the regular posters here have (or claim to have) working line dogs. That's not the kind of dog usually in rescue (though some rescues do see them from time to time, often as adolescents when first-time owners have an "oh no! what have we done!" realization). Most of what we see in rescue are "pet line dogs" who don't have the same intense need for stimulation and a "job." They still need exercise (walking, hiking, playing fetch in the yard, etc.), some training (at least basic obedience--which ALL dogs need), and a routine with clear boundaries in the home....which you are ready to provide. You should be able to find a good trainer whose philosophy matches yours for a basic 6-week course -- and they'll make you a better owner by teaching you specific techniques that help you manage and shape behavior.
Many of these adult rescues have already lived in a foster home where they were house trained, crate trained, and maybe even leash trained to be good walkers.
To be very honest, my favorite adopters are the ones who are most thoughtful and careful about worrying whether it's the right breed for them. They tend to make good decisions and be fully committed to whatever they decide. They work with our adoption team in an open, thoughtful way to help us find them the right match -- even if it takes a few weeks or months.
One tip though: many rescues will want to wait until your fence is up before placing a dog with you. The reason is that you're a first-time owner, so you don't have a history of keeping a dog safe without a fence. Don't be surprised if your application gets put on hold until the fence is up -- it's not an insult, but just a common protocol.