Hi Mick
I'd done club rides of around 100-150k, and a few sportives of just under 200. Then I entered LEL 2013. My friend asked why I hadn't joined Audax (I had planned on paying the extra £2 per ride, as a CTC member, to do some longer ones for training), so I signed up (5 years for 4) in March 2013, and did a 200. Did a 300 a fortnight after that, a 200 a week later, then a 600 (part DIY) three weeks after that. By the start of LEL I had done over 3000k of Audaxes in under 5 months (a pittance compared with the achievements of a lot of the others). Now into my third season, with more badges and medals to aim for!
Take it easy, and you'll find as long as you refuel, have the right kit, and sleep when you need it, you can do any distance. There is plenty of scenery on an Audax to be enjoyed!