If someone requests me to sit there dog outside what I stated as my radius, can I charge more?

I think that would be appropriate because it's 8 extra miles each round trip, and if you do 3 trips a day that's almost a gallon of gas (for my car anyway). Definitely let the customer know immediately, before the meet and greet. That will save you both some time if the customer doesn't want to pay extra. I would suggest $5 per day more.

As with anybody offering a service you control your price. Feel free to adjust it accordingly. Depending on how many times you will visit the dog per day/how far they are from you, you can add anywhere from 5 to 10 dollars a day and sill be competitively priced. Try researching any dog walkers in your area and see how they structure their price. Hope that helps you out:)