The time it takes to walk 15 miles depends on how fast you walk during that 15 mile walk.
If you walk at a very fast pace then you would cover a mile in around 11 minutes which would mean that you would walk 15 miles in 165 minutes.
If you walk at a slower pace then it might take you 15 minutes per mile to walk the 15 miles which would mean it would take you 225 minutes to walk the 15 miles.
Walking 15 miles takes a lot longer than it would seem.
15 miles seems pretty short when you're driving the 15 miles but when walking it seems like 500 miles.
Still if you can walk 15 miles per day it would give you good healthy and it's good exercise but I can only walk a few miles per day myself.