Welcome. Because your stride and your speed are unknown, it would be best to mark off a route to follow or use a pedometer. Your last minute of walking before you stop will be much slower then your start.
I have 12 inch tiles in my house. Using these as reference points, I can measure how long before my leg starts dragging (my left leg too). BUT it depends on the day of the week or whether I am having a bad day. Perhaps you need to average yourself over a week's time? Some days I will awake with my left leg dragging until bed. On other days it drags part way through the day (usually the latter half of the day.