Bear in mind that if you start in the upper parking lot of Magome at the end of the bus route (missing most of the town but I found Magome very schlocky and touristy), you'll have a shorter stretch of uphill and then a long gradual downhill to Tsumago -- a town which definitely feels authentic at any time of day, at least to me.

I know this because we were dumb enough to do it in the opposite direction!