I've never done a walk like that, but I used to do 3 mile walk a day for about 6 months. I built up to it by going as far as I felt like I could make it back to the house (choose REALLY comfortable shoes with good arch supports). Before I knew it, I was walking 3 miles easily within an hour & 1/2 & less eventually as I started 'power walking'.
I'm sure that you already have what it takes to get you through the whole 6 miles - dedication and love for your nephews! Good Luck!