It's human nature to put off things that are uncomfortable, and talking to your parents about their care can be just that. The obvious time to intervene is when you have no other choice, but so much stress and heartache can be avoided if you begin to get things organized long before it becomes a crisis.
Talk to your parents. Let them know your feelings and assure them that you are here for them and will be a willing participant in all of their elder care issues. Ask that they share their wishes and fears with you as well. Discuss the issues of guilt and being a burden and try to make them see that they raised you and were there for you and all you want to do is return the favor.
Encourage them to get help as they needed only, for now. You will see that this will make a much easier transition into full care if ever the time or need arises.
