Actually, the Radiance departs Seward at 9:00 p.m. - boarding will start at about 1:00. Anchorage to Seward is only 127 miles - how long it takes depends on which motorcoach you're on. Some do express runs in under 2 1/2 hours, others do a tour that takes up to 5 hours. I expect that you'll be on the express run, but you'll still want to board as soon as you get to Seward, which will probably be early afternoon. You may well have time to go into town after boarding, though.
Murray