Help with training for a team

Hi, I'm an 18 year old looking to compete on a local club team. I have never swam or played any sports before, and I have been training since mid-August. At first, I could barely swim 25 m and now I'm doing 50s. I have a personal trainer who's working with me, and the coach has watched me before. He told me that I need to work on endurance, because I can't repeatedly do 50s or even 100s with short rest. How long do you all think it would take if I trained extremely hard to do this? What do I need to do to make this happen? Thanks for the help