Some posters have told you, you are incorrect but have not stated why.
LACTATE in humans is a last chance fuel. It is alkaline and NEVER causes pain.
A medical professional would have to do a work up on you to know why you are tired.
Endurance and stamina athletes slow down and experience discomfort due to cardiovascular drift, muscle damage from pushing muscles to the limit. Sprinters and runners experience additional fatigue due to gravity. When we run, gravity is slamming us back to earth. Fatigue due to gravity is why it is more rare for runners to set multiple personal best in the same T&F as opposed to swimmers whom are more capable of setting multiple personal best in the same S&D meet. Runners take longer to recover than swimmers due to gravity causing runners additional fatigue.