training for a distance

I would need to know a more about your 5 mile swim and regular training to best answer your question.  Do you swim with a coached group of masters swimmers?  If so, then I would defer to your coach for his/her advice.  Where is your 5 mile swim going to be?  Ocean, Gulf, Lake, River, etc.  What types of conditions will your swim encounter?  Water temperature, waves, swells, current, marine life, etc.  How long are you planning for your 5 mile swim to take you to complete?  This will effect when or whether you will need to hydrate and/or feed during the swim.  Will you have a kayaker or other support team with you during your swim?  Is there an opportunity for you to practice together if you plan on having a support person?  Will you be wearing a wetsuit?  Have you practiced in the same wetsuit?  An open water swim is not the place to try new equipment out.  Practice with whatever equipment you will need for the swim (wetsuit, rash guard, open water buoy, etc).  There are a lot of aspects that should be looked at beyond just the distance.  As far as whether you need to swim the same distance prior to the swim, I would recommend that you do it at least a once, so you know what to expect from an overall fatigue standpoint.  More importantly, I would also recommend that you train in some of the same conditions prior to your swim if they are going to be difficult (waves, cold/hot water, currents, etc).  This is likely more important than the distance itself.  There is nothing worse than being in great shape for a swim and then finding out that the distance is not the issue, but the conditions are and that you are not prepared for 4' + waves, jellyfish stings, and 60 degree water (or whatever other challenges are created by the conditions).  

I would need to know a more about your 5 mile swim and regular training to best answer your question.  Do you swim with a coached group of masters swimmers?  If so, then I would defer to your coach for his/her advice.  Where is your 5 mile swim going to be?  Ocean, Gulf, Lake, River, etc.  What types of conditions will your swim encounter?  Water temperature, waves, swells, current, marine life, etc.  How long are you planning for your 5 mile swim to take you to complete?  This will effect when or whether you will need to hydrate and/or feed during the swim.  Will you have a kayaker or other support team with you during your swim?  Is there an opportunity for you to practice together if you plan on having a support person?  Will you be wearing a wetsuit?  Have you practiced in the same wetsuit?  An open water swim is not the place to try new equipment out.  Practice with whatever equipment you will need for the swim (wetsuit, rash guard, open water buoy, etc).  There are a lot of aspects that should be looked at beyond just the distance.  As far as whether you need to swim the same distance prior to the swim, I would recommend that you do it at least a once, so you know what to expect from an overall fatigue standpoint.  More importantly, I would also recommend that you train in some of the same conditions prior to your swim if they are going to be difficult (waves, cold/hot water, currents, etc).  This is likely more important than the distance itself.  There is nothing worse than being in great shape for a swim and then finding out that the distance is not the issue, but the conditions are and that you are not prepared for 4' + waves, jellyfish stings, and 60 degree water (or whatever other challenges are created by the conditions).