The amount of elevation change does matter, too. I am talking about both the difference between the trip elevation and his normal living elevation as well as the total elevation change within those 18 miles.
If you're not even carrying your own food/water 18 miles should be a piece of cake.
A trail on a hilly terrain is a completely different story.
I did half-dome in Yosemite many years ago, and if I remember it's about 7 miles one-way with ~4000-5000 feet of elevation change. But the trail is mostly quite good, and it's as much as I'd want to hike in a day.
For a two day trip, especially if the trail is rough, 18 miles sounds about right.