Setting Reasonable Goals

Hi all, I am joining a masters swim program as a beginner (I know how to swim, just not experienced or good at it yet). I don't know how to go about setting goals and I am hoping that some of you might be willing to offer suggestions. I know that fitness is #1, but if I don't have goals I'll get bored. The trouble is, I have no frame of reference for what my improvement should look like. For time availability, I can swim up to 5 times per week, 1-2 hours each time (although one hour is more reasonable, I can commit up to two if necessary). I'm tall and skinny and not naturally athletic. Right now I swim a 50 yard free in a minute, which means I'm in great shape for an 85 year old man. I am a 35 year old man though so that's not good. I can only swim at the end of a pretty exhausting day so that will probably negatively impact my abilities. Assuming working with a talented coach, what are reasonable and appropriate goals? Should I have a goal by the week, month, 3 months, or some other timeframe? For those who have gone through this before (going from non-swimmer to swimmer as an adult, no history), any tips? Approximately how long of dedicated training did it take to reach a good swim time? Thank you everyone who helps me out in setting some reasonable goals for myself.