Expect alot of mild scrapes on your toes and the tops of your feet as you learn the mat is your friend. Those areas tend to be softer and not used to alot of contact with course materials
While the tops of your feet do come into contact with the mat during seiza, they're not supposed to be moving across it so as to create scrapes. If they are, that's a sign that something isn't right.
To OP, I would say:
- Forget the "grabs" you learned in karate. This is a different style. Don't try to use karate techniques in your aikido class, and don't talk about how you have such-and-such rank in karate. Be a beginner.
- Be patient. The basic techniques of aikido take more practice to be able to execute effectively than the basic techniques of karate. Do what you're taught, don't try to modify techniques, just keep trying to do it the way you've been shown. It will come eventually, but you need to be patient.
- Be prepared to be sore as hell after your first class. Karate, properly practiced, strengthens the core, but it doesn't challenge core strength on day one the way that aikido does.