15 miles is a hike, but it wont tear you up after you do it for a weeek or 2. It only feels like a lot because many office drones expecation of amount of walking a day is anchored at "sit at desk all day." Appalachian Trail hikers start at 8-10 miles a day and work up to around 15 miles a day on a dirt trail over mountains while carrying 1/3 of their body weight! And they do that every single day.

15 miles a day is no big deal, to say nothing of 7 miles a day.