If you are after more of a challenge, and aren't interested in longer rides (why not? wrong sort of challenge?) then think about how your bike would compare with what was around when audax started. Most likely it weighs less. So get a heavier bike or add a brick to your saddlebag (there are other anti-weight-weeny options I'm sure). Or have fewer and more widely spaced gears (or, if you are on fixed, get a longer/lower gear). I've been reliably informed that there is a fairly high correlation between shininess/weight-weeniness (or not) and how early folks arrive at a control.
You will then have bragging rights about being closer to audax roots.
Distance brings a different set of skills, which I am not interested in... like how to avoid saddle sores, how to avoid your hands going numb, how to avoid collapsing at the side of the road due to sleep deprivation, how to avoid constipation, how to force feed yourself when you can no longer eat, how to wipe your bum when you have to go in the wild... It's all stuff that has little to do with riding a bike hard. I found a 600 was long enough, if not too long already. For example, a very fast 24 hour time triallist, is unlikely to be also at the very top end of a Transcontinental Race an viceversa, just completely different skillsets within the same "long distance cycling" label.
I have a 1980 bike, it's pretty much the same ride as my carbon bike, except I have slightly bigger gears. I think my PB on a 10 mile TT is very close. I don't think that going back further and learning to ride an ordinary would satisfy my hunger for challenge to be honest. Besides, old bikes bring in problems that you can no longer fix, due to parts being unavailable.