I used to think that running "slow" runs too fast meant simply being too tired to run fast runs fast enough. But it is a bit more complicated than that. If we don't run slow enough often/long enough we fail to develop our aerobic - mitochondrial - biochemical pathways, so we keep having to go anaerobic at too slow a pace so are forced to slow down even more. If you cannot chat in sentences then your slow runs are too fast.