It's all in the mind! Life begins at 65 (67, 70 or ...)! What a marvellous opportunity, and what vistas open up ahead! You have the advantage 'every which way' - you are now entering a new situation, with new opportunities - nothing is set in stone; there are the potentialities of a new start. It depends so much on attitude!
Remember: You are not yet 'home' (in the biblical sense) - you are still a pilgrim. Too frequently we hear talk of 'closure' when we should perhaps see it rather as 'transition' - one 'ending' leading on to another 'beginning'. First there may be need for rest and refreshment following times of stressful activity, for adjustments in reconnecting with family and friends, or for time to find a place to live.
Consider: For what purpose has the Lord called you back? What was the motivation that sent you out - and what now brings you back? There is only one Kingdom work, so you can look for similar ministries springing out of that same motivation - almost certainly not exactly the same, but ministries that express the same concerns that were worked out in your life overseas.
It hit home for us first when filling in a questionnaire: "What is your occupation?" ... answer ... "Retired"! That can almost feel like something of a 'put down'. Fortunately that word is not in the Bible, and it certainly is not a biblical concept - neither is it a genuine concept for those who have been busily working overseas.
So now you're 'retired' - or you have been told you are! But you're nowhere near finished! Sometimes it feels a bit 'scary', but actually you are merely entering on a continuation of 'whole-life discipleship'; and you will have much experience to draw on with which to help your friends, both in society in general and in the church, to learn that they too are on a journey of 'whole-life discipleship'.
'Home culture' has changed since you left - do all in your power to maintain your counter-cultural lifestyle; you come from a different lifestyle, so bear witness to the best aspects of that lifestyle. You will have an acute, and never-to-be-lost, awareness of the different values people place on things in everyday life.
Much of your experience of re-entry will depend on the level of integration and cultural adaptation that you have been privileged to enjoy while serving in overseas cross-cultural mission situations; at the same time, your experience will depend on how much of your roots in the 'home' culture has been lost - or has been maintained while you were away. They say that for every three years you live and work overseas it takes one year to learn to re-orientate to home!
Of course the parameters that controlled your structured life "there" have now gone; and of course there are choices that have to be made. Prayer and thought need to go into working out new situations and new disciplines. What are the indicators you will be factoring in to any of the important decisions you are going to have to make? What will take precedence in your decision making - family, accommodation or church? Rarely will all three of these considerations coincide. Being available for those special events involving grandchildren come quickly into focus. And how are you going to find out what is going on in the community around you, as well as the wider family, in order to make the best decisions?
If you are returning to your sending church, the leadership may well have changed and developed interests in other areas. Or, you may have moved away from your 'home' church, and may be 'the new kids on the block'. It will be very important to find a local church, preferably within easy reach, so that you can get yourself stuck in to the life of that church and benefit from a supportive fellowship.
One challenge ahead of you is that you will often find yourself slotted into the average Christian's concept of "missionary"; and you will find that few people will be really interested in your work. Mission has been your life and your joy for the past 'n' number of years, and it takes careful thought how to communicate a sense of 'mission' (in the widest sense) and to enthuse the Christian community to which you are returning. Be aware that it is quite possible that your special skills developed and used while overseas may well be adaptable to the 'home' situation; or they can still be used in different ways for short term overseas ministry - wise selection is necessary here.
And - how will you use your time now that prescribed work may no longer occupy so much of it? Look around you and intentionally notice and observe what God is doing in the society around you, and draw on those observations to see how you can help there - and how the church can help you. Whatever you may think of Cameron's Big Society, there is no shortage of opportunities for voluntary work (just look at your daily 'junk mail'!). Many organisations and churches rely on volunteers, a good number of whom are retired. 'For the Christian, every good job, paid or unpaid, has the potential to be done as an act of worship'. Never forget also that as a retired missionary you may have gifts or experience, or an insight about God that will be helpful to others, whether in a church or social or voluntary situation.
So finally! Continue to enjoy sharing in God's activity of building his kingdom while pacing yourself properly, giving due attention to 'anno domini' and remembering that parts do have a tendency to wear out!