When we are told to "never lag in zeal," we must not look for a feeling of spiritual energy.
Zeal is a feature of the presence and freedom of the Holy Spirit in the life of the Believer. This Christian zeal is the result of decisions made - decisions resulting in obedience to Him.
The zealous Soldier of The Cross is the one who does not allow periods of passivity. He simply decides what his priorities will be and he lives accordingly.
The zealous are not a saber-waving kamikaze warriors, charging on a fast horses. They are plodding Soldiers, who have looked up and have seen His Graciousness. His Love. His Mercy. His Majesty.
They have decided they will love, serve and obey. They are eager for divine correction. They remain steady because they can see eternity. Even when the body gets weak, purpose does not become diluted with physical limitations.
To the end, which is only the beginning!