Your yard will be fine. We live in a townhouse with two greyhounds. They sleep 18 hours a day. We walk them three times a day for 10-15 minutes at a time. By the way...we're 73 and 72.
They are definitely couch potatoes. Ours are fine with playing in the yard, but like all dogs they do love to go on walks. It can be a short walk. ;-) You should look into getting one! They will provide you with so much love and they truly convey appreciation for rescuing them.
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