My opinion and it's only that is, your terrier should never be out of condition. A few short walks to empty out and piss about should be enough, I keep all of my terriers off the lead, they'll do a few hundred yards more than me everytime with chasing each other or hunting up. Terriers aren't running dogs, they don't need tons of road work, if it makes you feel better about yourself and more confident in the stock you keep then more power to you.