I think that science lectures are probably the best place to find someone speaking continuously about a particular subject. The usually go on for about 2 hours, and include both lecturing, and a question and answer session at the end.
If I could recommend an accent, it would be
Christopher Hitchens. That's a video from the Atheist Alliance International by the way, so if you're not an atheist, or questions of religion don't interest you, you may find it heavy going. But he does have an awesome accent.