You can walk a JRT pup for hours (absoutely dont, obviously, keep it to 5mins per month of his age everyday until a year old) and they will still do that hun.

Thats just JRT's, they are terriers, made for ranging over miles daily and digging/hunting rabbit, rat etc.
But just carry on with ball games, frisbee's etc, go to the local butchers and ask for cow shin bones/marrow bones, or get him a frozen rabbit and defrost a little everyday for him gnaw on (raw food is great for his health, also!) to keep him content. Alot of the time chewing isnt from lack of exercise as some say it is, it is caused by boredom because people don't let on that these dogs crave heavy mental stimulation. Hide treats around the garden, give him a sand pit to dig in, line out a few coloured cups (so he can't see through them!) and put a treat under one for him to find. Take this time to get him used to the collar being on him, teach him to walk to heel (all my dogs are taught to walk at heel before using a leash....they NEVER have a lead put on them until at least 16 weeks old), teach him recalls etc.
Good luck =]