Back to OP, I'm in a similar position myself. I'm hoping to make a trip in August (WHPV 2015, Maasmechelen,
http://www.hpv.be/wk/?q=en which will require riding about 330 miles in 5 days, so similar daily distance to the planned Audax (and then the same back after the event). I've clocked up over 3000 miles in the last year but not much in May and early June. My usual longest ride is a grocery run of just over 27 miles which I normally do in about 105 minutes. Yesterday I rode 42 miles at a similar pace and, I don't know if it was psychological or physical, but I felt a massive drop in energy levels at about 32 miles at a bit over 2 hours.
It was clear from yesterday's experience that other systems become stressed on longer rides. I don't have issues with bike fit as I'm riding a recumbent (velomobile). As advised above, I would really strongly advise doing some longer rides to find out what potential problems might occur for you - you'll be so much more confident and relaxed about doing the actual ride. I'm a similar age (59) and did manage a 70 miler last year and a 50 miler in April. However, I was pretty well keto adapted for both those rides and didn't get the energy slump I had yesterday - which raises a whole other topic ....