before you decide on a JRT make sure you understand a few things:
They should NEVER be left unattended with other dogs, they tend to have aggression towards them. Some JRTs will be aggressive enough to never be aloud to interact with other dogs (if you rescue an adult over two years chances are you'll already know if he's this bad so you will be able to avoid it, if you get a puppy then it's a gamble, either way he should still never be left alone with other dogs).
They should never be left unattended with small children, they will not put up with rough handling.
They need LOTS of exercise for both the body and mind, this means your one walk a day plus plenty of playing in a fenced in yard off leash each day and training to work her brain. (is your yard fenced?)
These dogs will go after small game like squirrels, rabbits, chipmunks, cats etc. so should not be off leash in an unfenced area.
Have you read this stuff?
http://www.therealjackrussell.com/breed/index.php
http://www.therealjackrussell.com/breed/jrprofiler.php
I'm sure JRT owners (we have a few who are very active here) can help you more to be sure you are choosing the right breed. My main concern for you is energy and dog aggression, they are very active dogs and just a walk will not be enough. I don't think more walks are needed but lots of ACTIVE (makes them pant hard) play is needed as well as intense training. They are very intelligent animals so training should be fun for you and the dog.