First, you need to do a mock test. Before I am able to give you any meaningful programming or conditioning suggestions, I need a benchmark and context. Are you trying to max (or close as possible), or just pass? It takes 2-4 weeks to gain or lose cardio/endurance conditioning, and you are running out of time!
I’m going to take a guesss and assume that it’s because the main adaptive response is to get more efficient at carrying oxygen, not necessarily building anything which takes a considerable amount of time and energy (like building muscle and getting strong). having to get good at a mile and a half seems pretty trivial and doesn’t need much training to pass requirements.