Shocked and sad

Some of my best childhood memories are of the time spent in Michigan at my Grandma and Grandpa's house.
It was a six-hour drive, so we didn't visit more than a couple of times a year and when we did go, it was something special. I remember my grandpa as a tall, strong man. He was kind with an easy sense of humor. He died of cancer when I was in grade school and my memories of him have faded. Now I'm left with more of a feeling about who he was than anything else.
He's more of a person in photos and as I sit here, I can't think of any memory that really reminds me who he was. All I have are vague recollections of him playing Superman with me, greeting him when he came home from work -- and worst of all him lying in a hospital bed watching my baptism on a VCR because he was too weak to attend himself.
Now I'm terrified because my grandma has a tumor in her colon, likely a recurrence of the cancer from 20 or so years ago.
She has already said she doesn't want to go through chemotherapy or radiation again, so it's probably a matter of time.
I have many more memories with grandma -- memories that really stick in my mind. But how long before those start to fade?
I want to hold on to them as long as possible, so excuse me if I reminisce a little in the next couple of days.