Losing Petrol

After visiting my Great Aunt Bernie, I travelled home in my old jalopy. The car was so old and battered, I was stuck in second gear. This meant that I could only travel along at a steady 30 miles per hour and managed a paltry 20 miles per gallon of petrol. At the start of the journey I had placed exactly 10 gallons of petrol into the tank. I knew though, that the petrol tank had a leak and lost petrol at the rate of half a gallon per hour. As I arrived home, the car stopped because it had run out of petrol and I only just made it. How far was it from my Great Aunt's to my home?