2 man Kayak

I have not Kayaked in a long time but live on a lake. Recently I picked up a 2 man kayak (Fibreglass) at a yard sale. It is around 12 feet long but the holes are too small to allow me to slide myself inside. I am 5’10" 175 lbs. The roof rests also fit inside the compartment and appear to indicate that you are supposed to paddle kneeling up. Am I missing something. I have not tried it yet since I suspect such a high center of gravity will make it inherently unstable.

Yes you are missing something
You bought what is called a C-2.

C-2s are decked canoes which typically have small, round or oval cockpits, and are designed to be paddled by two people, generally wearing spray skirts, and using single-bladed paddles.

They can be great fun. By a book and read up on canoe strokes, if you don’t already know them.

What is a “roof rest”?

Roof rests are I suspect
pillars or bulkheads inside the hull supporting the deck, or a bulkhead between the cockpits.

C-2s do have a higher center of gravity than K-2s due to the kneeling posture of the paddlers. They often tend to be a bit wider in compensation.

Although they might be said to be “inherently unstable” in the hands of two reasonably experienced paddlers they are quite stable in a dynamic sense, since the crew paddling on opposite sides provides outrigger support, and a strong low and high brace is available on both sides (as opposed to just one side, as in a C-1 or OC-1).