just like working out it gets easier as you train, for quite a few serious runners 10 miles a day is par for the course (100mi training weeks are not that uncommon).

My dad played for a living in the 60s and his band played TWO shows a day (afternoon and evening, so the schedule was get up around noon, breakfast, show, lunch, show, dinner, sleep) 6 days a week during the summer, it's just like any other type of physical labor and listening to him it definitely beat working 12 hour days in the fields which is what he did before...