Ch. 7: The Governor's Hall
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"She was now of an age to run lightly along by her mother's side, and, constantly in motion from morn till sunset, could have accomplished a much longer journey than that before her."
In other words to the first answer, Pearl was probably little since she was a child. And because Pearl was so little, you can imagine she had "baby legs" or small legs so she had to run a bit to match her mother's pace. Just imagine how you were when you could first walk perfectly fine and now you had to catch up with your parents when they're shopping or going on a walk in the park.