If you feel fatigue in your legs in these types of events, then I believe it’s a muscular endurance limitation. The intensity is high enough that you’re calling on fast-twitch fibers to do the work, but FT fibers endurance is naturally low.
For more information on muscular endurance training, click the Muscular Endurance forum category on the right-hand side.
WRT muscular endurance versus lactate threshold versus VO2, it’s good to remember that nothing works in isolation. All of these things play a role; there’s no single factor at fault. It also varies from person to person. From your description, it sounds like the main factor for you is muscular endurance.
I hope that helps.