A dilemma

Mild dilemma faces me this evening. I've been invited to a friend's wedding reception, where I expect to know only the bride and possible one other person slightly. My invitation was for me and a guest, however my guest can't make it and it seems too short a notice period to enlist another.

Getting to the venue will be a half hour train followed by a cab for a few miles. I've been unwell this week which has lead me to cancel the three nights' previous engagements, in a bid to regain health and also to ensure my ongoing vitality for tomorrow's planned trip to Glasgow, not to mention next week's Trip To New York.

I am leaning towards unfortunately missing my friend's nuptial celebration, however I'm unsure of the etiquette of the apology in such a situation. Do I draw attention to it ahead of the event itself, or explain myself after the fact?