Pace

pace
verb, intransitive. To walk at a steady and consistent speed, especially back and forth and as an expression of anxiety or annoyance.
verb, transitive. To measure a distance by walking it and counting the number of steps taken. To lead another runner in a race in order to establish a competitive speed.
“Pace oneself”: to do something at a slow and steady rate or speed in order to avoid overexerting oneself.
To move or develop something at a particular rate or speed.

Marcy had been shopping for fifteen minutes before she became aware of the man who was pacing her as she moved up and down the aisles. At first, she assumed it was just her imagination, but soon acknowledged that she had acquired a shadow.

Still thinking she could be wrong about him, she continued to consult her list and fill her cart, keeping the dark-clothed figure in her peripheral vision. He stayed about four yards away, stopping when she stopped, moving when she moved. Her sense of alarm increased, and she was glad there were so many people in the store.

What about the parking lot? She had parked some distance from the entrance and it had already been dark when she entered the store. Should she ask for the manager and tell him she wanted an escort to her car? Should she call Jim at home and ask him to meet her here? He wouldn’t be happy about that, but it would be preferable to having his wife abducted by a serial killer. She mulled her choices as she worked her way from Produce to Dairy.

In Frozen Foods, her patience snapped. She spun on one foot and looked directly at the man. He was ordinary-looking, in jeans, a t-shirt, with a jean jacket over that. He had dark hair and wore glasses. Even though the two of them were close to each other, he stared into the ice-cream case before him, refusing to meet her gaze. She stood her ground, determined to keep looking at him until he responded in some way. She studied the part in his hair, the pale cast of the skin on his face, the stubble on his cheeks and upper lip, the set of his mouth. If he did attack her later, she wanted to be able to give a detailed description of the creep.

He could only tolerate her scrutiny for one or two minutes. Suddenly, he turned away from her and hurried to the end of the aisle. It was only then that she noticed that he had no cart and nothing in his hands.

Definitions adapted from The New Oxford American Dictionary, Oxford University Press, Inc., 2005 (eBook Edition, copyright 2008), and from Webster's Seventh New Collegiate Dictionary, G. & C. Merriam Company, Publishers, Springfield, Massachusetts, USA, 1965, depending on which is more convenient to hand.