when do you know that your dog is tired of walking?

When spring finally comes, I will start riding my horse every day. We start out slowly but it is not long before we are going out for a couple of hours at a time. Susie travels twice as far as the horse, running ahead and coming back, chasing squirrels, etc. I would really like to know how far she travels. I can use the GPS and tell how far I have ridden but I am sure I would have to at least double the distance for Susie. I have taken her on some Rides where we leave at 8 am, have a lunch break, and get home at four or five in the evening and she is going the whole time, plus swimming in every water hole we come to.