Retirement is a major transition -- and one reason I wanted to reprint your letter is because I hope it will make other people who are nearing retirement stop and think about this important step. Many people don't, and (as you've observed) they end up feeling bored or useless. But it doesn't need to be that way.
How should you view your retirement? First, accept it as a gift from God. That may seem hard for you to do right now, but remember: Every day of your life is a gift from His hand, and just as He has been with you in the past, so He'll be with you in the future. The Bible says, "His compassions never fail. They are new every morning; great is your faithfulness" (Lamentations 3:22-23). Thank Him for being with you, and seek His will for your future.
Then ask God to help you discover some new path of service. It may be a part-time job; it may be volunteering in your church or a local homeless shelter; it may be some other way of helping others. Don't waste these years, but ask God to show you new ways to use them to help others as long as your health lasts.
Finally, make it your goal to deepen your relationships -- with your family, with friends, and most of all with Christ. Make sure of your commitment to Him, and then make it your goal to "Come near to God and he will come near to you" (James 4:8).