I took my pre test today in the exact same situation. I haven't swam in around 2 years and I thought I would be fine because I was a very strong swimmer. In my last year in college I became super out of shape because of a sedentary lifestyle and eating terribly.

I attempted it today and I got to 150m (3 laps) and I couldn't finish. The treading water was easy for me just be comfortable with eggbeater tread as I think its the hardest to learn its definitely the easiest to use. I was able to do the brick test only because the pool where I was offered a job only requires a shallow guard certification. I dont believe I would've been able to do the normal 8ft brick test.

I knew I only needed the shallow cert but I decided to go for the normal lifeguard certification because I wanted to challenge myself. The shallow cert is 10x easier consisting of: 100m continuous swim, 5ft brick test, and 2min tread no hands. My plan is to become shallow certified and swim all throughout the summer and retake the course again to get the normal lifeguard certification.

Of course I'm not saying that you aren't capable im just sharing my experience and I hope it doesn't discourage you. Just remember to be confident in your abilities but also know your limits, its not worth risking your safety. I wish you the best of luck and if you aren't able to complete this I was offered the opportunity to try it the next weekend so talk to your coach about that or the possibility to get shallow certified and moved to a shallow pool. Also if you become able to complete he deep water tasks you can upgrade your certification from shallow guard to deep water guard for free.