Do it while you can, in my opinion. There are too many uncertainties for you to plan, with certainty, to do this hike 20+ years in the future.

Many of my friends (I'm 62) have had things happen that would prevent them from doing the hike if they wanted to at this age (e.g. arthritis, death, cancer, heart ailements, kidney disease=dialisis(sp?), etc.

Everyone hopes to be fit and healthy later in life but no one can count on it.

Perhaps you could do portions of it, a little each year?