Here is the passage again:
While thunder boomed in the distance, Margaret started off on her daily
five-mile run. Confident that she had plenty of
time before the storm arrived. Wet hair and clothes plastered to
her body proved that she couldn't outpace Mother Nature.
The second part is a subordinate clause fragment. It has a subject, she, and a verb, had, but the subordinate conjunction that keeps the clause from expressing a complete thought.