I would plan on getting there before show time. The parking can sometimes be tricky in downtown Alexandria (lots of meter parking), so I would allow time for hunting for a parking spot AND then walking AND still getting there 20 or 30 minutes before the show.
I'm probably going to wear lighter armor (mail, surcoat, maybe padded cuisses), and will be in all of it before driving over, so I don't have to finish armoring in the cold on a busy street.
I don't recall how long it runs. I want to say 90 minutes or so. But yes, since it's the only show, they won't chase you out.