Train from Anchorage to Denali leaves 8:30 AM and gets to Denali at 4 PM. You have to overnite, and take a morning tour into the park. That is 8 hrs. Then train home. Even driving is 5 hours. There is construction this year near Willow, so that will slow it down more. Plus they are putting in a new overpass at the intersection of Palmer & Wasilla, so that will be slower going.
But if you have two days, you can do Talkeetna by rental car. It is actually closer, (as the crow flys) to Mt. McKinley, and offers ALL that most folks are looking for. GREAT flightseeing over Mt. McKinley. River adventures, both rafting and riverboat on the Talkeetna & Susuitna rivers, as well as fishing. You can also horseback ride there, and hike. The bears are usually out at night on the river - if you do an evening run. Here is a local company if you want free advise:
http://www.1anchorsaway.com/alaska/alaska.htm
Your going to see lots of marine life and glaciers on your big cruise, so consider something different for your days in advance. Seward is just more of what you will see on your cruise.