I am just about to take part in a 750 mile event on Sunday which I have been training for since the end of last year. Unfortunately, the run up to the event has been a bit of a disaster with an impacted wisdom tooth removed 6 weeks ago which has been causing problems since and then a back problem that is still ongoing but getting better. I indulged myself in some Powrlink pedals, as have always ridden Speedplays, and before the back problem it was L 47% R 53% which has now been reversed, I estimate my power is down by 18% (left side/leg problem and R a constant) . This is going to push me up against the time limit and I was wondering if anybody knows of some magic juju that might improve my chances. I know this is a total shot in the dark and I am not expecting an answer but as they say, if you don’t ask you don’t get 
Can’t say I have any real advice except to make sure that you stretch frequently during the event and try to do it before you actually experience any discomfort. Like with fueling, once you get the pain it could be too late. So be proactive and stop at regular intervals to take a moment to stretch and change your position.
What have you been doing up until now to help fix the back? Lots of things can cause it but try doing some gentle stretching on your hip flexors, glutes, hamstrings, etc. in the next couple days.
Idk if you already do this or not but strength training can really do wonders to combat back pain. Many times back pain is a symptom of weakness somewhere else and you back is compensating. So deadlifts (and it’s variations) along with generally getting stronger can help a lot. Obviously this won’t help you in the next 3 days but worth thinking about going forward.
Is it back pain and where?
Do you have weakness anywhere?
You mentioned your legs power differences. Can be pressure on a nerve or a disc issue. I have both.
You should think about not riding and taking care of your back.
Me being the “never say never” type ;
With my disc issue I experience a sharp pain in my lower back when moving a certain way. It takes about a month to run its cycle. Usually peaking in about week then slowly going back to normal. I have ridden events. Usually 1 Advil and 1 Tylenol 30 min before the start. When done I have had to unclip and put the bike on the ground to get off. Unable to walk upright for 10 min and a painful drive home.
Nerve pressure is another issue and if ignored can lead to loss of the muscle receiving signals to fire. Over time this can lead to, as in my case, atrophy of my thigh and was unable to lift my leg while lying down. This took 18 months to heal. Not something you want to experience.
I question 750 miles on Sunday. Multi day event.