They are kind of like mosquitoes, but smaller and more numerous. On the plus side, their bites are much less itchy. I'm not camping, so I'm only mildly annoyed by them. Still, it's tough when you can't stop for a rest without getting swarmed.
Sandflies and uncertainty about where I would sleep made day 4 tough, even though I only went 56km (33mi). Today, day 5, went all right too. Longest day so far at 83km (50mi), and I made it over Lewis Pass, the highest point, so it only gets better from here!
The mood swings on this trip are remarkable. While I'm going uphill, I want to pack it all up and go home. On the downhill, I'm pretty sure I thought "I would be mostly content about my eternal future if heaven is just like this forever." In flat places, my mind goes everywhere, occasionally returning to "eh, I'd kinda rather be sitting drinking a coffee or something."