I will track steps with a step counter (free app called "Step Counter - Pedometer" which probably installed malware on my phone) and expect this will be around 56-60 miles of walking. I plan on attempting this on a treadmill* (I bought this one in January).

Progress updates on activity + intermediate goals/milestones are available in a google sheet . Activity since Jan 1st is logged in there and I will try to keep that reasonably up-to-date. As noted in the google sheet, I have a provisional target completion date of Saturday March 30th (Why? Because I live in a Catholic area and we get Good Friday off).

*[updated with new info on 2/24]

One notable issue with the treadmill is it is visibly inclined in the "0" (non-inclined) position, which I validated by making some measurements (see google sheet for details). The incline is ~3.7%, which would mean that I would be ascending ~11,300(!) feet during the challenge if I leave it as-is . Others have done their challenges outside, which would naturally include some gain/loss, but probably nothing close to this much. I have been doing trial walks mostly at that default 3.7% default grade, but plan on nearly leveling the treadmill prior to the actual 120K challenge (I will err on the side of leaving a +grade of 0.1-0.2% so it isn't unfair).

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