Some people really love to talk
One of the best courses I had in college was an English class on the 18th Century Novel. My professor assigned one book per week and would ask one question per book for which we were allowed to answer on only one typewritten, double-spaced page. The problem was that the question asked could be answered more easily in 20 pages than in one. That was the beauty of the course. I learned to write succinctly.
The same techniques used for writing succinctly can be applied to speaking succinctly as well.
While most novice speakers would probably prefer a 10-minute presentation to one lasting 40 or 50 minutes, many speakers find the 10-minute presentation harder to make than a longer version. Some people really love to talk.
The answer to cutting down your lengthy presentation is to ask yourself this one question: what must I say to get my point across? By means of an outline, list what you need to cover. Once you have those bullet points, build on them with care.