I thought the previous comments were cool, but I'm gonna add something that works really well: train with light weights and lots of reps doing the movements you actually use in muay thai, like kicks and punches. That builds up local muscular endurance and you can keep the intensity going way longer without that crushing fatigue you get from just doing generic cardio.
The thing is, traditional cardio works your aerobic system, but your muscles specifically are still gonna get tired fast in sparrings if they don't have that endurance. Mix interval training (those guys were right about that) with low-load strength sessions, like maybe 3 days a week. Makes a huge difference in just a few weeks.