Possible to improve 30min distance w/only 30min sessions?

Hi, This is my 1st post. I have been a fitness swimmer (off & on depending if I lived near a pool) for nearly 30 years, since my mid-20s when I taught myself to swim a mile freestyle non-stop. (I had a few pointers from a local swim coach back then) My times have always been around 30 minutes, give or take. I am now retired and have a year round indoor pool. It's short... 56 feet I think. 48 laps= 1 mile. Yes, it's a lot of flip turns. I still do dry land, weights, pushups & chin ups etc. But I don't like spending more than 30 min in the pool. Typically, it's mostly freestyle, with a few backstroke & kickboard thrown in...all nonstop & flip turns. I went all out on Friday, and still fell just short of a mile :-( My question: Is it unrealistic to expect to improve my time without spending more time in the water? No Masters around here (Camano Island Wa), but I did hear a local pastor used to coach a Brazilian Olympic team... ps I was just reading some threads here and found out the S curve stroke is no longer advised. Ha! News to me. Maybe that's my problem.