Suppose there were a 400km calendar Audax organised as a figure-of-eight with the start/finish in the centre. That's perfectly feasible, isn't it? Anyone wanting to do a 400 but unsure about the full distance would surely find that an attractive option. But maybe the more experienced riders would think, rightly enough, that an important part of the essential challenge is diminished.
Such rides already exist. I think there's even one with clover in its name, with a route much as you'd imagine. Hang on... Yep, this is it:
http://www.aukweb.net/events/detail/17-19/ - that's only a 200, but there are longer rides that follow a similar format. The Fairies 300 in June is broken into 100km and 200km loops.
I don't think such routes diminish the challenge - you only get credit for any audax ride if you complete the full distance that you signed up for - ie you can't enter a 400 and then downgrade it to a 200 mid-ride; nor can you upgrade from a 200 to a 400 if it turns out on the day that you fancy some extra miles. If anything, they are more challenging because you have to find the motivation to go back out again once you return to HQ. You may not find that a problem, but many do - I know this because it's been a regular topic for discussion for at least as long as I've been doing audaxes (12 years).
I would also say there is no point doing an audax if it's not a challenge - biting off slightly more than you can comfortably chew is part of the deal, you just have to be careful not to bite off so much that you choke on it. And part of that challenge is ensuring it's not too easy to give up at any point. Pretty much every ride has its low points but in my experience, you can usually ride through the rough patches and recover - many is the time I've felt like giving up at 3/4 distance, but pushed through and finished the ride feeling on top of the world. If you think you might want to give up at some point and get a lift back to the finish, you might be better off finding a nice sportive with a broom wagon service.
I entered the Brimstone 600 a few years ago. My build-up hadn't gone to plan, so I wasn't in ideal shape, and it was appalling conditions on the day - torrential rain from the start, barely above freezing, very strong headwind along the south coast... and on top of that, it's an
extremely hilly ride. By the time I reached the Priddy control (390km), I was on the verge of being outside the time limit, but I'd already been riding for 24 hours by then so I needed a break. After a couple of hours of fitful sleep, I was looking forward to the cooked breakfast the controllers were very kindly offering, but within minutes of finishing it, I was outside hurling it into the hedgerow. I pressed on, but was seriously suffering, walking up even the gentler climbs, and stopping regularly to throw up. Even so, I was in two minds about whether to abandon, partly because it's so bloody difficult to get back to Poole from northern Somerset. In many ways, carrying on riding was the easier option. Ultimately, though, I decided to divert towards the nearest station (about 20km off route, iirc) and make the tortuous train journey back to Poole. I still regret that decision to this day - I'll just never know whether or not I could have recovered and finished the ride (although less than a third of the field did make it back inside the time limit, iirc). Definitely intending to go back and give it another go one day though.
I'm doing the O&C 300 tomorrow and riding 65km to the start, which will mean setting off from home at about 2.30am. Yeah, it's going to be physically tough, but you know what? It's still only a bike ride. I know and believe that I
can do it, and that's half the battle won already. The really difficult part comes at around 240km when the route passes within a few miles of home. If I'm feeling low at that point, I will find it hard to motivate myself to ride all the way back to Meopham.
