It is definitely do-able, just depends on how much you want to complete the ride.
My first Audax was a New Forest 100 in 2006 then followed by a SR in 2007 to qualify and complete for PBP. Many people packed that edition very early IMHO due to the weather rather than the distance.
As for the last couple of years, audaxing has taken a back seat (is that a very bad pun??
) mainly due to family & work commitments but I have completed and DNF'd (bike failure) the Dorset Coast 200 recently but last year I was determined to ride LEL. I even worked out a schedule of rides to build up to it but mainly because it didn't need the qualification my participation in these rides vanished but LEL came and I completed it.
There were times, just before Thurlby, when with a failing set of neck muscles and a rear wheel giving way that I thought I may have packed but the energy, enthusiasm and all-out helpfulness of the volunteers re-invigorated me, so a big thanks to them.
So it is really all in the mind....
Go on you know you really want to do it......