Here are two ways to build up the length of your speech:
- Add in more PARTS* for each main point
- Add more main points
The advantage of adding in more PARTS for each main point is you don't have to remember as many main points. You can give a 60 - 90 minute speech on 2 to 3 points by adding more PARTS for each point. In calculating how many PARTS you need, consider that each PARTS takes about 7 - 10 minutes to tell.
Making it longer,
* PARTS is mentioned elsewhere in the Public Speaking Library