If remaining on payroll through age 55 is critical in terms of your benefits and you have concerns that your employer might discharge you early if they learn of your plans, then I would suggest that you keep your cards close to the vest until you turn 55, then provide notice and plan to work for a couple weeks more. A couple weeks or even a month is not much to ask to leave on good terms after a long career.
Alternatively, you could provide notice in early April 2013 and let them know that you intend to leave efffective April 26, 2013.
Depending on how much you trust those who you work with you could always "informally" let them know about your plans so that they can begin planning for your absence.
If there is any question in your mind that they might not do the right thing, as always, look out for number one.