Ultimately... it's your decision to make... with regards to if you are suffering or in pain.
Suffering just means you need to push your body past it's limits and deal with the discomfort in doing so.
Pain means "If I keep going I'm going to get injured".
If what you are experiencing is pain, then you need to not race. Races will always be there. Your health is precious and should be taken care of above all else.
Yes, it's disappointing... you have spent so many months planning... waiting... organizing... visualizing the race. To have to cancel is upsetting.
So you have a couple of choices:
1) Mentally call off the race and don't go.
2) Go to the race, and tell yourself that you will start, BUT if you are in pain, you will stop.
A lot of people can't pull off #2, and if you think you can't... then go with #1.
BUT you paid your money... you won't get a refund... so maybe just go and see what happens?
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"