On an island under imminent threat can all you can do is run from one end to the other?

Yes, it does. ;O

If I knew what the threat was, I could probably think of a more productive reaction.

If you are in imminent danger, running isn't going to make it go away.  Enjoy the time you have doing whatever it is you like to do.   
Why not open the best bottle of scotch or wine you have, grab your significant other/friends/ pets/ and use the time you have with them? :)  That's what my husband and I did when we lived in Key West. 
 

you could go side to side...haha
or maybe dig a very deep tunnel
to hang out in.