Veterinary surgeon here (I originally made this account as a throwaway but it seems to be my 'vet advice' account because I don't want anything on my main account that could ID who I am, anyhow...). I would argue that practically every dog needs two hours exercise a day. Dogs aren't made for being cooped up in a house all the time and no the garden doesn't really count. They primarily need to get out to sniff and to engage their brains (think exercise yard at a prison seeing the same things all the time vs actually being allowed to leave and experience new things). It doesn't mean you have to do one giant two hour walk after work - it's actually better for the dog if you split it so maybe half an hour to an hour in the morning (especially in the summer - get out early before it gets warm), half an hour at lunch and an hour in the evening.

About german shepherds - yes, in principal I agree with rescue dogs but a huge amount of GSDs have behaviour issues. I'm probably going to get downvoted for this but in my ten year + career as a veterinary surgeon I have met a LOT of shepherds and this is based on experience. I have met a few really lovely ones who are friendly, but they tend to be a very nervous breed and can be very very difficult to live with (think being fearful and fear-aggression to any new situation/person) and especially if you have children make damn sure you know what you're adopting before you do.

If you haven't had dogs before I'd recommend starting with an easier breed in terms of temperament, and the need to keep their brains occupied. I agree with the poster who suggested a rescue greyhound - tons of these dogs needing good homes, when they run they run but they're generally easy going and like to laze around. (only thing is most of them are ex-track dogs so absolutely not if you have a cat or anything else small and fluffy that you care about. Some need to be walked with a muzzle because the drive to kill anything rabbit-shaped is a big one