"There are 2 ways a runner can perceive the impact of running beyond the threshold pace. First, you will incur an oxygen debt. This will manifest itself in a feeling of shortness of breath; that your lungs are not large enough to suck in all the air you want. Second, as lactate accumulates you will feel a 'burning' sensation in your muscles.

In general a highly trained, elite, runner will have a marathon pace just under their lactate threshold; for less conditioned/slower runners it might be closer to their 10k pace (or about the fastest pace they can maintain for an 1 hour)."