As you get used to it.

So I've been running for a few weeks now. At first it was hard work - I could just about manage 3 kilometres without stopping, but I'd feel pretty dreadful during the run and cough for the rest of the day. 

Gradually it became less hard and I could extend the distance to 4K, then 5K  a couple of weeks ago. 

But here's the thing: it's started to feel harder again.

This is normal, but I don't entirely know why. Perhaps it's because it's not so overwhelming that I'm able to notice all the hurty bits much more. Perhaps it's because I'm not fit and strong enough to run easily that it's hard to break out of the plodding. Or maybe I'm just not driven enough to over-ride my body's demands to take it easy. 

If I can keep running for a few more weeks like this then I should break through, but if I stop for a while then it's back to square one.