I agree with dan@deb, give them a chance. Viking is a high quality cruise line. Most cruise lines don't give you the details about your flight until a lot closer to the trip then we would like. There could be a ton of valid reasons why it is taking so long. While we have gone on our Viking cruise yet, I'll answer your questions with what I've been able to learn myself.
You don't have to do the tours at all, if it doesn't sound interesting and you should be able to depart from the tour if you are not finding it enjoyable, but as you said, make sure you tell them.
Are they boring, all depends on what interests you. It's Europe, so it's going to history and the such. I've heard almost all positives about the tours and the guides themselves (only complaint I heard was that someone basically wanted an advanced tour for those who already new about the place). But that's not to say the style will be to your particular liking.
Are you to young? God, I hope not since both DH and I are younger than you. In general the pax will be much older than you, but that doesn't mean they are fun. We are going with my parents. My mom usually stays up drinking later than I do. So age has nothing to do with being fun. And usually, you can go of the boat a night and hit any bars in town.
Remember, river cruising is VERY different from ocean cruising. It is more laid back and focused on your destination. No beer drinking contests or Las Vegas style shows.