If she wants to, let her.
How old is she? If she's a young pup, it may be best to give her a few breaks, but an adult dog that is willing and capable of walking that distance can.
I know a lot of dogs, including pretty small Chihuahua's, who LOVE to walk and will forever. I think walking is so good for both human and dogs. I walk with Jackson, typically speaking, 2 miles every day. But sometimes we go as far as 5. His little Yorkie girlfriend, Nala, is about 6lbs and can easily go 4-5 miles.