You should be swimming three times a week if possible to build up your paddling muscles. Most of your time in the water is spent paddling, and the more swimming you do, the easier it will be for you to catch waves and paddle back out.
Push-ups are also important as they are an integral part of the pop-up. The more push-ups you can do, the more times you'll be able to pop up to your feet from the prone position, and the easier it will be.
Core strengthening exercises will help with balance and stability while paddling, getting up, and riding waves. Try a pilates class or add in some crunches to your daily routine. Whenever you do push-ups, you should do as many crunches.
Flexibility is also important and key to being able to get up to your feet. Try a local yoga class to begin to limber up for your trip. Simple stretches like the forward bend (stand with legs straight and lean forward until your hands reach toward the floor by your feet) will help immensely.
Running or any other aerobic exercise will help you stay out in the water longer. Even just 20-40 minutes of cardio will help.