Why Do My Legs Sink No Matter How Hard I Kick?

I used to kick a lot but I was just getting exhausted. Then I was told: “Don’t kick. Save your legs.” So I stopped kicking… and my legs still sank. I also heard: - “Kick like you're kicking a soccer ball” - “Save your legs for the bike & run” - “Kick from your glutes” (but no one told me how) I followed it all. Still slow and my legs continued to sink like a rock. Next came the gear: - “Just use a…