My husband said something about he is thinking of getting me a Fitbit (a really fancy step counter) for my birthday. I think those a pretty expensive so I asked him whether it would really be worth it: "What would I do with a Fitbit?" So he told me that I should challenge flat earthers to step contests.
He is probably right. I daresay it would do all of us more good than having debates.
That's a cute idea but the discrepancy in the length of stride even for the same person, is greater than any difference found between expected distance and actual over any reasonable straight course. You'd be walking for days on end with no conclusive results -- except perhaps consumption of energy.
A fitbit can count how many steps you take and/or how many stairs you climb (or descend) but it has no way of knowing what distance you traveled over the ground. I know a guy who can't figure out how to get store discounts using his smart phone, but he loves to save money using coupons. He drives the cashier crazy asking questions about using the store app.