Our expert says:
It's not so much that you are running out of oxygen as that the effort of breathing becomes limiting just because it gets so hard - your perception of that effort is what causes you to slow down, and in your case, it happens before your legs fatigue. The answer to this is to train optimally, and to do training that will improve your respiratory capacity - I would suggest doing regular speedwork sessions - on the track if possible, where you do reps of a few hundred metres, with recovery periods of a minute or two in between. For example, 8 x 400 (at faster than your 5 km race pace), with 1:30 rest in between is a good session.
Once it happens in the race, you have no choice but to slow down - but, in training you can progressively work at it so that it becomes easier and easier.
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