Beta girl.... GJ on getting in the pool. KEEP AT IT. It will get better.
As far as a good time? Well, you could check out the race results from the previous year to get a good idea of what the times were. Then, you can swim that distance in the pool and time yourself to see how you will do.
For my 1.2 mile swim, I did it in 40 minutes. But, I was also doing once a week swim workouts that totaled 2200 to 3000 yards each session. AND I was a competitive swimmer... 20 years ago!
Just do what you can... and feel good about being about to do it.
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"