I’m sure I can’t be the only person who gets issues pre event; be it injury, sickness, equipment failure or just plain bad luck.
In theory next weekend I’ll be doing a 220km audax but I’m currently suffering from a flare up that makes everything a bit uncomfortable (maybe some British understating going on here) so not ridden this week. A week wouldn’t effect my fitness but if it’s like this I don’t think I can face 13 hours in the saddle. At what point would you call it, a week, 2 days…
Really depends on what else hangs on that decision. If it doesn’t matter to anyone else if you turn up or not, you can wait until the day of, or the night before. If it’s an over-subscribed race, and your spot could go to someone else - call it early and give them a chance. If there’s travel arrangements, hotels to cancel etc, go by the cancellation dates. If mates depend on you to go, give them an early heads-up, so they can arrange something else.
I would just ask if you think you might have fun and what’s the long term damage you might cause. You’re going to be in the saddle for 10+ hours.
You’ve been looking forward to this for a long time as the big event, I would probably push through and at least show up. Maybe go out and do some fun hard/short efforts if you’ve been training the long endurance stuff to shake some muscle fibers loose. (2 days before you shouldn’t wreck anything)
This is the answer.
Will doing this thing add to your life or take away from it?
Net positive: go
Net negative: stay on the couch
If you think the problem is going to still be there and make you miserable, then I wouldn’t go do a miserable ride. It also depends how much worse it may get and if that’s something to factor. If you’re happy to take a few weeks off for the problem to subside after, and no permanent/super long term damage is expected, and you can enjoy it, I’d totally do it.
only you know the problem, what rides on the event (travel, time, etc) and what the short and long term consequences may be.
If you can give it a few days and see how a short ride goes, that might be ideal.
Either way, just realize you will make the best decision you can with all the information you have available to you at the time. Once you make the decision, just remember that to help avoid any thoughts of regret.
A few weeks ago I was on the fence about attending a major race, but leaning towards not going. Was quite smoky in the area and travel was significant, and I decided not to go. Two days before the race, I smashed an outdoor hill effort with a significant PB, and for a couple hours considered trying to get there anyway, as I was confident I had the fitness to contend for a win in my age group. By the end of the ride, the feeling had worn off, and on race day, there were only slight regrets but I was confident I’d made the correct choice as I was refreshing results page. 
I hope your decision isn’t too over-thought.
I had similar issues yesterday, we have a world of stress at the moment in our lives, plus after spending my working days over the last week stuck in a van with no air-con I felt tired and demotivated, I was down for a 100km audax (120 actual) and to make things worse the clutch on the car started giving us issues last week, so in order to get to the audax it meant riding an ECE (extend calendar event) which turned it into a 210km ride with just shy of 3,000m of ascent.
Long story short, I still went, but I knew that there was a bail out option of cutting the ride short at various points and getting the train home, in the end it was the right choice, had a great but very tiring day out, wonderful scenery, some good company in parts and the buzz from doing something physically challenging, plus a couple of decent coffees and ice creams along the way.
Unless you know it’s going to make your issues worse, then I’d give it a go, make a bail out plan though and if it feels too much or you’re not enjoying it, use it. I think it’s most often better to try and fail than not try at all, if you don’t go, you’ll most likely spend the day feeling a bit down. Given it’s an audax you’re doing, it’s absolutely none competitive, so just go and enjoy the day, or at least as much as you can manage.
Day before, hehe!