I adore my dog, she's made my life better in a thousand ways every day since I got her and I wouldn't trade her for the world!
Having said that, there's no way in hell I'd be able to have her if I didn't live with someone who was able, willing, and happy to walk her three times a day - sometimes more - even when it's raining or snowing, drive her to vet appointments, sort out pet daycare for her when needed, take her to training classes, pick up her heavy bags of food from the shops, get up to let her out first thing in the morning, and just generally do the things that I am unable to do. There are so many hidden aspects to owning a dog that take up a huge amount of energy and even after literal years of research and planning I still underestimated how much work she would be.
I think adopting an older dog if you can is absolutely a great idea for lots of reasons, not least offering a dog a second chance at a happy life, you also have a much better idea going in of the energy requirements of the specific dog and their personality - if they're happy to chill out on the couch for most of the day or want to be occupied elsewhere even when not on walks etc.
Greyhounds are wonderful dogs and so many of them need homes but be aware that even the smaller framed ones are very strong, I know someone who was pulled off her feet when her dog took off after a squirrel and broke her collarbone. Something to consider if you are not always steady on your feet (which I know I personally am not).
A lot of people suggest small 'lap' or toy dog breeds to people who have limited ability to do long walks but don't be fooled, small dogs can be some of the highest energy breeds! Make sure to do your research and, if rescuing, ask lots of questions to get a good idea of the specific dog you're interested in. Small dogs are great because they're easier to travel with, lift up or down and carry, they eat less, do smaller poops, can fit on your lap or beside you on the bed/couch, but they're still a lot of work. They can also be more prone to loud or frequent barking which can be an issue if you have sound sensitivities.
I have a one-year-old miniature poodle. I wanted a low-allergy, low-shedding, small-to-medium sized companion dog and I definitely got those things, but even after all my research, looking carefully for responsible breeders of healthy good-tempered dogs, discussing my needs with her breeder and visiting the puppies several times to find a good match, she's still turned out to be a loud alarm-barker, has very high energy, and demands all of my attention. People are often surprised when they meet her thinking that poodles, and especially the miniature size, are quiet, calm, and dignified. Not my girl!

But then again, I wouldn't have her any other way.