Just a quick piece of advise:
Your listeners are conditioned by television to the point that they really only have an attention span that is, in total, about the length of a TV episode (22 minutes). Along with this, they are used to segments that are about 10 minutes long. This means if you don't do something about once every 10 minutes that jars them back to what you're doing (like a commercial break), they're going to be drifting off.
I was reading recently that the average American adult typically gets distracted at work about once every 2 minutes. What's worse (and I've found this to be true in my case), if they don't get distracted in 2 minutes, they will actually find a way to distract themselves. There were preachers of the past who could use lots of dry logic and long quotations, reasoning for hours and hours to a captivated audience. Those were marvelous times, but they aren't the times where we're living.
It is worth doing something to change directions or pace about once every ten minutes. This is also why I prefer not to make lots of bland introductory comments. I know that the moment I begin to speak, my clock is already ticking. It's better to engage them while they're still listening, rather than to use up their attention spans announcing what they already know about the weather.