The acerbic moderator of a news program once concluded an panel's interview with the famously verbose Senator Hubert Humphrey by noting that perhaps they had enough time for one more question but certainly not for one more answer.
A key talent when answering questions from audiences or panels is knowing when to stop. In general, a three minute answer is better than a 10 minute one if it is clear that you'll be glad to elaborate. Toss the ball into the court of the questioner so it is that person's decision (and so he or she will take the blame) as to whether you will devote more time to the subject.
This does not mean that you slide past some crucial context that must be conveyed for clarity's sake. That context, however, should be given in a short paragraph.
Preparation, of course, is the key. Rambling presentations are the result of inadequate preparation and its child, unclear thought. Thinking out loud is fine for brainstorming sessions but a presentation that seeks to persuade is usually not assisted by injecting new issues.
Far better to leave the listeners a little hungry for more than stuffed with information and ready for a nap.