I studied Latin for 5 years. This may sound weird... from my experience it is extremely difficult for native English speakers to learn it if they lack solid English grammar skills. My teacher would get so frustrated with us that she had to spend most of her time teaching us English before we could understand Latin sentence structure, declensions, etc. Use the rest of your time this summer to brush up on your old fashioned English skills... as in learn to diagram a sentence, if you don't already know how. If you have good grammar skills, you'll be fine.
After studying Latin, learning Spanish and German were quite easy... although I remember so little of them now.
Btw, Latin helped me ROCK the SAT... as in I missed 1 question on the Verbal section. It helped me get a perfect score on the ACT.