This is quoted a lot, and it's important. Still, when you are in the situation, you don't know (for sure) just how long the train ride is going to be. You are playing it in the stew of the most powerful human emotions --hope, fear, love, and a desire to say one more nice thing to your grandchildren. It is a day by day existence.
Everything in life would be a lot easier if we could make our decisions with foreknowledge of the outcomes various choices would bring. Among other things, I'd quit indexing/rebalancing and turn into a stock picker and a dirty market timer in 5 seconds!