How can I add a time duration field to posts

I’m working on a blog where the majority of the posts are about video (but not every post). For the posts that are about video I want the post header to say the usual Post Name with the Post Date but then and have an extra descriptor field called showing the Duration for the video. My intent is to use css to target the extra field for presentation

Batman vs Predator Fan Film 12 mins posted June 13, 2011

how would I go about achieving this? Is there a plugin that does it?

Was also wondering if I am going about this the wrong way. Maybe a plugin is not needed at all? If the first line of my post was <p class=“duration”>12 mins<p> then I could target the new field with css and float it right.
Would this be a better way to go about this? (and avoid possible plugin clashes)

Batman vs Predator Fan Film posted June 13, 2011 12 mins

See: Custom Fields « WordPress Codex
This sounds like what you are shooting for.