Do you have any In/Out points set on the timeline? Specifically check to see if you've inadvertently set an Out point on the timeline that falls about 15 minutes from the end. I've done this accidentally before.
If you're putting HD source footage onto a DVD you're going to be compressing the footage a lot. You're going from HD (source footage) to SD (the DVD). Not necessarily bad, but keep it in mind. Burning a blu-ray would allow you to retain more of the quality (but you'd need extra gear as Mac's don't have Blu-ray enabled drives). Use Compressor and drop the "Best Quality 150 minutes" preset on there for DVD. That will get your entire file to fit on a DVD.
Ryan Holmes
http://www.ryanholmes.me
vimeo.com/ryanholmes