Friends confided in me they are eloping this summer?

The other evening, two friends of mine told me they are eloping in a couple of months. They told one other person (in my presence) but no one else knows about this. I was asked to keep this to myself. While I can respect their request, they also asked me to attend the event, which is in the middle of my camping trip. Granted I'm only 75 miles away, it means that I will have to make up some lie to my friends, who are also friends with the engaged couple, about where I'm going, etc. Then there's the thought I get when the others eventually find out about the marriage and look to me for answers as to why I didn't tell them. How do I handle this situation when confronted with it?