“Spiritual drink” and “spiritual food” is that which is given by the Spirit of God. Galatians 4:29 speaks of Isaac as being born “after the Spirit.” In other words, God intervened in the life of Sarah and miraculously brought about the birth of Isaac. What the Israelites ate and drank in the wilderness was not produced according to nature; it was supernatural. Furthermore, that spiritual food and drink “followed” them.
Christ was the invisible and mobile power behind the sustaining food and drink in the wilderness, which the Israelites failed to see. There is a divine omnipotence that preserves us and sustains us, and that “accompanies” (KJV and NKJV “followed”) us through life. Too often we “lightly esteemed the rock of [our] salvation” as the Israelites did (Deuteronomy 32:15). We enjoy the physical benefits of His preserving power, but we miss spiritual significance behind it and never really enjoy that accompanying presence of God in our wilderness.