Lactic acid is one of those things that can slow you to a stop. It's a waste product made by the inefficient burning of sugars in the muscles when you exercise them. It builds up in the muscles and it hurts.
I'm quite good at ignoring lactic acid for a while - but from experience, on my first attempt at a marathon distance - pushing on through can lead to collapse.
I ended up lying in the road in crying and moaning, intermittently struggling to my feet, and shuffling and staggering about, moaning like a zombie in sweaty running gear, before collapsing again. Some very lovely people at an old vicarage off the road let me lie down on their garden furniture for a bit, and gave me a lift back to town.
Now I treat the signs of lactic acid with a bit more respect. I try to make sure I'm well hydrated before I run, and take isotonic drinks with me, and energy drinks if I'm going for a long run.
Anyway, today, it seems that all the boozing over the holidays hasn't left me in good shape. I got about a kilometer into my run - I was aiming for a half marathon distance today - and felt my shins start to swell with the nasty nasty pain.
So, I turned back and only put in about 4km - a tenth of a marathon.
I suppose this means I have the opportunity to get on with more writing instead.