Practice, practice, practice. If you audio or video tape keep practicing until you can lengthen your speech when timed.
Tips for doing this are breathing (at intervals) as suggested by someone above. Make sure you put your pauses (breaths) in where they do not appear out of place. And do not put in too many if you cannot moderate your inhale to just be a mere pause. If you cannot control your breath at these pauses, it actually works opposite (in many cases - not all) by causing you to breath too much. Which can cause your speaking to become breathy and often one will INCREASE their speaking speed in order to finalize their speech so they can get a "real" breath. So, it is good tactic to use. But, make sure you have the breath and mind control to not cause yourself to hyperventilate or concentrate on the breaths so much that you actually cause increase in speaking speed when you wish to slow it down. Do not make large inhales to pace yourself, just use the breath as a pause. Otherwise you just have rapid speaking with a bunch of stops between which does not help your understanding for your listener. Not really.
Another tactic is extending (without over ennucuation of) vowel sounds in words while speaking. Is frequently used to slow speech down in rapid speakers (or stutterers). As a tool to help slowing speech down to become more understandable for a listener.
If you cannot tape (audio or video) yourself (the best method to really KNOW how fast your speech sounds to others) to see/hear what you sound like and the speed, do the following:
Get an ordinary egg timer. Or stop watch if you have one.
Have someone neutral (meaning not a pal or family member who will just say "was fantastic - no changes required") be present to listen and give you real input on whether speaking is too fast, too slow, etc.
Get your speech notes prepared. And get ready to give your speech.
If you use a set timer like an egg timer, set time for a long time out (way past what you know it will take). Then, give your speech as your normal pacing currently allows. You KNOW it is gonna be too fast - based upon what you have already said here. But, that is not important. What is important is that you have a baseline time in order to know where you are and where you need to get to. Does not matter if your speech will have a certain time frame or not. Just that you know your starting speed.
If you have a stop watch to count up to give you end time - hit the counter to start.
Give your speech. Determine how fast it went.
Once you have baseline time. Add 3 minutes to the end time as your goal for your speech. DO NOT ADD words or content to your speech to make your time get to the end time. Slow your speaking down until you can comfortably (and with consistency) give your speech at that new time. If still too fast, continue to practice adding 2-3 minute increments WITHOUT ADDING CONTENT until your speech is delivered in a manner that is well paced, not too fast, not too slow and wherein (if you use breath pauses or vowel extension) nothing is seen by your listener as a coping tool. Then, practice the hell out of it once you have perfect time.
Before your speech - when your anxiety and worry over giving it is highest, meditate and remember you have it down. You have your time to hit. Check your watch as you walk to podium. Check your watch during your speech to make sure you are on target (or close) and know you will be fine. For, by the time you have stepped to podium? You could give your speech in your sleep - perfectly. Just do it.
You can, I know it.
Good luck!
Christine