I've done a grand total of three audaxes. I like them, from the point of view that they're more relaxed than other organised rides. But I still just like picking my own route and running to my own timescale.
First I did my road bike developed a terminal problem the night before so I quickly bought some slicks for the MTB and did it on that - just 60km I think;
Second one I foolishly didn't check out the route elevation and took the fixed on what turned out to be a 'hill audax' - 100 miles and I was knackered;
Lsat one was the Tour of East Lothian last year (which didn't go up the killer Rigg due to poor surface caused by the poor weather) and I was on the geared Kaffenback, with a pannier stuffed with food. I hadn't ridden any great distance in months, but the gears, road bike and plentiful food whenever I needed it made allllll the difference.