Need help structuring a 2 day swimming boot camp

Morning all!

I am looking for some quick swimming training advice.

This weekend, I will be going up to my Dad's place in the bush, and have basically all-day access to a lake for some swimming practice.

Now, this will be my first shot at "structured" swimming in close to 20 years. I used to be a very good and efficient open water swimmer, but have ZERO clue of my current level. So let's say I am starting from scratch.

This will be my first swimming training getting ready for my first tri in September. Starting next week, I will be swimming about once a week (at lunch time). So I would say that this weekend will be a "reality check".

Any suggestions on how I could structure my swims for Sunday (afternoon possibilities) and Monday (all day possibilities) and turn it into a nice short swimming boot camp?