Success on G day = Commitment to train + Determination to go beyond + refusal to quit when it hurts
1-Shoot for minimum of 3km swims.
2. Do at least one long swim per month of half the weekly distance
3. Always try to do more than you initially set up to swim
4. Get used to over delivering
5.Train in OW as much as possible
6. Swim in a variety of OW conditions. The tougher the better
7. The more you relish adversity and exert yourself in training, the better time you will have on G day
8. Success on G day = Commitment to train + Determination to go beyond + refusal to quit when it hurts
9. Push your limits at least once a week (either distance, or speed, or external elements) Every time you go beyond what you previously thought possible, you will get stronger both physically and mentally and you will be better prepared.
10. Go beyond on a regular basis. If you get tired and frustrated in practice and you stay in the water, it is unlikely you will prematurely quit during the G swim.
Swim4Good 
The above works, believe me and it’s pretty close to my approach. Having a goal really helps to maintain your focus. Conversely, not having a defined goal allows you to make excuses, “why bother?” Message to myself – find a new challenge.