Any Help Appreciated...

Hi, so I am training to do my SLSGB (Surf life saving course) the exam is this August.

I have to swim 16 lengths (standard pool) in under 7.5 mins. I am a terrible swimmer.

I bought the TI book, which has helped an awful lot. It has given me a stroke that means I can now swim 8 sets of 2 lengths each ( I have to catch my breath before continuing for between 30 seconds to a minute) a VAST improvement from 2 months ago, however I still have an awful long way to go. My average time for 16 lengths at the moment is 20 mins.

I swim three times a week.

I need to shave 12.5 minutes off my personal best in just over two months.
Any Advice?