I have never done this particular exercise, but counting paces is a surprisingly accurate way of tracking distance. I can achieve less than 10% error over distances of a kilometer or more, which is way better than my estimates. Too bad you can't use a GPS since they are still more accurate.

Lay out 100 feet on level ground, walk it several times, and do the math to get your average pace (left foot to left foot).

There are actually formulae for accounting for steeply sloping ground. I believe you drop paces descending and add paces ascending, but I have never used them.