Okay then, my rough estimate is something like 10 km just using my finger to measure it, but thinking more scientifically we could say that the journey started in the middle of the bottom-left square and ended in the middle of the top-right square! That means that we walked something like 9 km up and 4 across so we could look at Pythagoras Theorem (Pythagoras Theorem asserts that for a right-angle triangle with short sides of length a and b and long side of length c, thus: a² + b² = c²).
So we can use google calculator to calculate the distance by using the following sum: sqrt((9*9) + (4*4))*0.621371192 and we get 6.11979651. Now we multiple by 2 and get 12.239593. Or a little over 12 miles, but c isn't a perfect straight line so we could feasibly round that up to 13 miles I guess. Which I think you'll agree is a bloody long walk .