I think that there can be a lot of variation in this, and certainly how much weight you're carrying is a big factor. I was 54 when I hiked the AT, and started in pretty lousy physical condition --- I had had foot surgery 3 months before and for most of the intervening time wasn't able to walk much to train. I averaged 12 - 13 miles per day the first week out and generally increased it from there, and once out of snow did 20+ pretty much every full hiking days apart from in the Whites.
That start worked fine for me; for someone else a slower start might be more prudent. It's hard to quantify, but I'm sure that the fact that it wasn't my first long distance trip helped too.