Training intervals

What's the best way to build endurance for 200 stroke events or mid to distance free? some people at my pool tell me to do tons of repeats with only 5 sec rest if 25s, and only 10 sec rest if 50s or 100s. this doesn't work for me if I'm doing stroke or IM. My form breaks down after not that many repeats and I have to add more and more effort only to get slower and slower. ( and get less and less rest - double whammy!) i know some people will do 20-40 repeats of 25 at race pace to develop a 200 but what should be the work to rest ratio? i also know there is a science behind all this. Different intervals to train creatine phosphate energy system (ultra short) anaerobic energy system, and aerobic. Sometimes I think it might be better to give myself longer rest, like 20 sec on a 50 stroke so that I have time to clear lactate and each 50 is cleaner. But how do I train my body to clear lactate more quickly so I can actually complete a 200 stroke with quality. People say do lactate threshold sets, but again I don't know what that really means for me in terms of work to rest ratio, and what intensity of effort in the work part? i just did a 200 fly as 3.46 even though I was able to pull off a total swim time of 3.28 as a broken swim in practice coming in under 52 per 50 @ 1:00. I didn't think that was much rest to make that much difference but apparently 8 seconds is too much to be realistic i think i am I am beating myself up thinking I have to workout more and more to gain endurance but maybe I am just overtraining because I'm not making the improvements I'd like to see for how often I practice. any suggestions for me? thanks in advance!