I should probably the last person to give advice on swimming, but I’d do a long workout (up to 90 minutes if possible) with threshold intervals and then speed at the end. Check out this type of training from the build and specialty phases of the mid volume plan. Then you get some speed and some more continuous, relatively demanding swimming every week. Oh well I might as well paste one here:
Long base swims are nonsense in the pool, and nobody that I know does them, unless they’re testing their 1000 m or 1500 m, in which case you do have to do a long swim to check your long swim time 
even in open water you can try to do 10-minute intervals, or even shorter, with an aussie exit to get you used to the galloping heart rate in T1.