I reckon you can easily do double your "normal" mileage without too much trouble, as long as you remember to eat right and keep your fluids up. Audax is not a race (quiet at the back there!) and if you can be comfortable enough on your bike for ten hours you will easily do a 100. Speed irrelevant.
Careful though, they are addictive. On no account get sucked into that "radonneur round the year" nonsense :-)
