Relax Sue, and stop feeling guilty. This is part of life and we all go through the same thing when our loved ones begin aging. They went through it, your kids will go through it, and their kids will go through it.
My mother and father are also in their late 70's and not very well. Dad lives in Missouri and Mom lives in N.C. I spent a few days in Missouri with Dad at Christmas and last weekend in N.C. with Mom.
They understand your situation and certainly don't want you to feel guilty. Just call them often, talk to them, send pictures, etc. and get over there to visit when you can. That's all you can do.