Don't try to fill 90 minutes with talk. Encourage dialogue and questions.
Steer discussion toward topics where you have knowledge and are comfortable.
If you don't know small group don't try to come across as an expert.
Good advice.. 30 minute speeches, laced with a little humor that are concise and to the point will leave your audience wanting more rather than sitting there thinking "are they ever going to shut up?" Following the presentation get the audience involved with questions and as soon as they begin to taper off, close and shut up.. Don't try to drag it out to a specified time.
As a minister I can tell you there is nothing worse than one of us that only has 15 minutes worth of material delivering a 30 minute sermon.