What time of day is your wedding? If it is mid-afternoon and they are having very warm weather, maybe I would consider hiring towncars or a van unless they are dressing fairly casually, and like Wendy said, maybe look at the forecast the day before and see if it's supposed to rain and then consider transportation. It wouldn't be a problem to at least get cabs if not towncars even that late.
Otherwise, I think it's a very nice walk. When I was at my planning session a couple weeks ago my sister and I timed the walk from the building whitehall is in back to the wedding pavilion. We made sure to walk at a leisurely pace and look around and it took 10 minutes. We thought it was a very pretty walk too, on a winding path with plants and scenery.