8lbs isn't THAT tiny. My friends 4lb and 6lb yorkies go on 4 mile walks easily with us. Now, they've been doing this a lot since they were young so they're accustomed to it. And the 4lb one HATES the cold so we don't usually take him when it's cold. You may want to start out with < 2-3 miles and build your way up gradually. However if she seems like she is doing okay, she probably is. Dogs usually let you know when they're tired or had enough. Jackson is my bigger boy, he's nearly 18lbs, but he easily walks 4-8 miles no problem (as long as it's not hot out, he hates heat and humidity).
I would just start smaller just to see what her limits might be. Bring water, a few pick-me-up treats for energy. And see how it goes.
Dogs love to walk and these guys are terriers after all. It's good exercise for all of us!