Although I'm not at that age, from what I hear what is helpful to senior organists are a few things:
1. Enjoy every moment on the organ bench. Always remember that practice is a privilege which we have to be grateful for. Sharing your skills with others to make their lives better is another privilege. Keeping this in mind, organ playing should be an activity you would simply treasure and miss if you couldn't practice.
2. Take your time and don't rush. At this stage of your life don't push yourself too hard. Although learning process might take longer than earlier for you, you are still making progress. Also don't attempt to play very fast. Start your practice at the tempo in which you can avoid making mistakes and stay in control. The general techniques for good practice habits still apply here.
3. Play only pieces that you really love. There is no point in practicing music someone else given to you that you don't enjoy or has little musical quality. The same could be said for the dry exercises unless of course they are meaningful to you and lead to the advancement of your skills.
4. Make frequent breaks in your practice sessions. I generally advise people to take rests, relax, and stretch about every 30 minutes or so. For senior organists, the breaks could be even more frequent. If you feel tired after playing for 15 minutes, there is absolutely nothing wrong with that. Take a short break, walk a little, stretch or lie down with your eyes closed and drink some water. Then you will feel refreshed and will be able to practice for at least another 15 minutes.
5. Imagine master organists of the past who might have been your age. For example, Johann Adam Reincken (baptized 1643-1722) was said to have lived until 99 years of age and was still active as an organist in St. Catherine's church in Hamburg at the beginning of the 18th century.
However, baptismal records in his native town of Deventer indicate that Reincken must have been born much later - but still he was almost octogenarian at the end of his life. By the way, Reincken was a key figure in the formation of J.S.Bach as an organist and composer.
Do you have any special tips that you apply yourself in your practice which would be of value to elderly organists? If so, share them in the comment section below.