Would definitely agree with kitty, it sounds like too long for a single workout and pools make it very easy to overdo it. Water is very low impact which is great, but even just freely floating in a pool engages nearly every muscle in the body. You might not feel it but everything is working almost twice as hard as on dry land, your body is trying to keep warm as the water lowers your core temperature even in a heated pool, forces of gravity maybe overcome by buoyancy but the resistance is that much higher. It makes swimming a great exercise to get in and stay in shape, but it’s killer when it comes to dealing with fatigue and weakness.

I would strongly suggest cutting the time you spend in a single session in the pool way back, and give yourself plenty of breaks in between. You might even do better to simply do a lap or two and then rest out of the water for a bit before going again. Taken slowly you can still get an hour’s worth of swimming in a day without having jelly legs, but a far more realistic goal if swimming just to exercise would only really need to be 30 minutes in a day.