Dirty.
Serious answer:
How I feel about an audax depends on the ride. I'll feel different after a leisurely 100 km in good company compared to a solo DIY 300.
On the longer distances (
>300 km) I enjoy getting into a routine of riding, focusing on the next instruction and thinking about my next target, be that a certain junction or a control. At that time nothing else in the world really matters and the crap from the week can be forgotten. In the past I've equated it how one may feel during meditation and the sense of peace increases with the length of the ride. Of course there are black moments, but I don't focus on these parts of the ride. After the ride I enjoy the sense of achievement, and also the disbelieve when people ask what you did at the weekend. It's nice doing things that people believe are impossible.
Audax also gives me a chance to see new parts of the world, and also at silly times. Cycling along at dusk or night is a fantastic experience as one can see the world from a different perspective. Of course one could do this without riding an audax, but recently a lot of my othernight rides have been on audax rides.