Suggestions on Tapering for LD Ocean Swim

Hi friends, I am trying to get information from any of you who have done some very long ocean swims. I have one planned that will be 18 miles. I am reasonably confident at the training required, although any suggestions for that would also be welcome. I plan on swimming upwards of 40 to 50 miles a week for it, regularly. I also plan on a long swim once a month (10 miles) and two weeks for recovery from that distance until my body understands that this will be a regular thing. On the 10 mile swims, one way will be with current, the return will be against it (can't wait!!). My biggest question comes to the tapering for the 18 miler. How soon, what distances, etc., etc. Any advice, other than seeing a mental counselor for my "temporary insanity" at choosing something like this at age 60, would be helpful!!! Donna