Alaska is huge. The shorter the amount of time available, the more planning that person(s) needs to do ahead of time. No amount of time will allow you to see all of Alaska in one lifetime, IMHO. Give a lot of thoughts to what you and your traveling companions want to see and do. It sounds like the OP has come up with a workable plan for them, with the time available. Fairbanks is about as close a town to the "old time Alaska" as you will find. Lots to see and do but you have to provide your own entertainment. Much of the state visited by most tourists is not unlike Disney. Show up, hand them your credit card and they will entertain you for some period of time. LOL A week day tripping out of Fairbanks will give you a feeling for the Interior of the state, then on south as outlined. I really don't care much for the Kenai area with all the changes in crowds of people over the years. We do go down for a few days to visit friends in Homer but then back north. Most of the day long boat tours are outstanding, from personal and other's experiences, out of Valdez, Whittier, Homer, Seward, etc.
The cruise ships only give you a passing view of South East Alaska. It is said that if you drive all the paved roads in Alaska, you will have seen about 5% of the state, throw in all the dirt/gravel roads, and you will have seen about 10% of the state. To see more, a person has to spend time on boats and airplanes.
For many visitors, a short first time visit with just inspire them to return, over and over again, as you can tell from some of the comments, it is addictive to many of us.
While a week or ten days in the state, followed up with the train trip and the cruise ship, will be less than optimum, it will be so far ahead of not going, etc. Some of the first or second time visitors will try to convince others that the way they made their trip is the only way to do it and enjoy. They don't realize that about 99.99% of all the RVers that go to Northern Canada and/or Alaska have just as good a time as they did.
But decide what you want to see and do, then plan your trip accordingly. A lot depends on where a person lives and where they have traveled previously. As a person raised on a ranch in flat land, dry, Oklahoma, anywhere with water and mountains really impresses me greatly. Folks that grew up in Seattle, not so much so it seems. LOL