Originally Posted by Soylent Green:
Since I'm NOT an athletic trainer and have no particular related expertise, take this for whatever it might be worth...
In general, I would look into sprints and other quick burst activities... much moreso than running distances or swimming. Run and hop the football bleachers... Run the bases... Explosive type movements.
+1000
It's sounds like you didn't really train in the off season. With 12 days to go it's a bit late to get 4 months of training in. However, there are a few things you can do.
1) start your baseball activities with a dynamic warmup. This will get your body ready for activity. Carioca, high knees, butt kicks, frankenstein walks, arm circles, walking lunges, ladder drills, etc will get your body going and dynamically stretch your body. Many of these drill have a short sprint element added at the end of the movement (i.e. 10yrd sprint at the end of a 20yrd high knee movement). A nice finish to this is a series of 30yrd sprints; 1st at 50%, 2nd at 75%, 3rd at 100%.
Now you're ready to throw, bat, take infield, outfield, etc.
2)Sprint and quickness exercises. Google SAQ training. Baseball is a series of explosive movements interspersed with periods of inactivity, it's not an aerobic activity. It's somewhat similar to football. So look to interval training, running form training, assisted/resisted sprints, parachute drills, pushing weight sleds, etc. These are useful for baseball and will help with your conditioning.