A few things will help with your endurance. One is to establish and perhaps increase your base mileage - the miles you run every week. The rule of thumb is to increase your mileage by no more than 10% per week. Around 20 miles per week should be enough to get you through the 5K non-stop and should help improve your time.. Two is to increase your long run of each week. Work one run each week to 5 or 6 or 7 miles and you will know you can comfortably finish a 5K. Three, like cannon1229 said, is to know your pace, what pace you are capable of holding for 5K. Starting out too fast really affects your pace in the latter stages of a race. On the other hand, if you start a little slow, you'll find you have more left at the end.

Len