You only got out of the car to stretch your legs and rest 15 minutes every 5 or 6 hours of driving? While I admit I have also done that in the past, it's a very dangerous thing indeed to do, even if you just jumped in on the driver seat. Makes you prone to mistakes, longer reaction times and to misjudge situations. There are regulations in Europe on just how much professional drivers (eg lorry and to a less extent taxi drivers) can drive every day, for very good safety reasons. Even if you think you're doing alright please do make longer and more frequent stops, you might find yourself involved in a situation where reacting half a second earlier can save your life and many others around you.