I'm, not saying that the Sighthounds need a ton of exercise, the problem is having the fenced area to get them to run, and keep them in (the Saluki high jumping comes to mind...).
The average backyard is not big enough for one to really get up to speed and stay at it. The ease of that 5minute run gets less easy when you need to drive 15 minutes to a dog park, worry about other dogs with that thin skin, drive home, etc. The happiest sighthound I've seen was a Greyhound let loose in a baseball field, but that means dodging the town police, animal control, and people generally wanting to mess with you.
We have a large fenced yard for the area, but our dogs run out of room relatively quickly when they get going. It takes Lexi (beagle/greyhound) about 3 seconds to cross the yard, fewer than 10 strides. I would want a bigger fenced area before taking in anything faster.