I really don't understand why this is a suprise to anybody. Even an out of shape person could make their way across Europe in about a month if they were dedicated to it, and a casual explorer could have made it from Turkey to Britain in just a few months. Since we ARE a naturally curious species, it's almost unbelievable to think that there wouldn't have been explorers back then trying to find out what was over the next hill or beyond that next valley. They probably had a reasonable idea of where the liveable areas were in about a century, so a growing population would have naturally expanded across the continent relatively quickly.
Europeans fully colonized the much larger North American continent in under 400 years, and we had massive mountain ranges, burning deserts, and a huge population of hostile natives to contend with. The first inhabitants of Europe had far less land and far fewer obstacles to overcome.
Human technology at that time was simply not advanced enough to overcome all the obstacles.