Katherine,

Sounds like a great challenge. Tough to train hard with only 4 weeks to go. How much training are you doing weekly at the moment? This will help me determine what you can do in the next few weeks. My best recommendation is to treat your swim as a 'jog' and not a race. The key is to find a pace where you are comfortable maintaining. Maybe 75% of your max speed. If you are comfortable you can swim for greater distance.

I did a 10,000 Meter open water swim this past summer, and the longest swim I did prior to the event itself was 7000 meters. My typical workouts the last six weeks was 4000 meters maybe 3-4x a week. Then I did a 6000 meter swim and later a 7000 meter swim. The 10,000 meter open water swim took me approx 3.hrs 20 min....should have been 3hrs but we had a current in our face at the end.

If you find that comfortable pace, you'll be amazed on how far you can go. If I were you try and find a Coach nearby and consult with him on your specific workouts.  Don't worry about stopping if you feel like it, nobody said you can't take a breather once in a while. Good luck.

Gerry