Pay attention to qwjack, Steve! The only thing I would add, to make it slightly clearer, is that when you take a distribution, it is taxed at your ordinary rate at that time. This can be very nice, if you are in a lower tax bracket after you retire.
SteveH said "i want to sell $210k of spy now (dec 2020)."
That withdrawal alone will put him in the 24% bracket if he's married filing jointly, 35% if a single filer.