swimming for an old(ish) swimmer but a new tri'er

I would say that swimming and running are my strong points, running i have been working on separatly for a while now, before i was thinking about tri's so i am pleased with my current trainning for that. For swimming, i used to swim all the time, but had a big break, but now i am back into it and can do 750m in under 15mins freestyle (i don't know if thats good or not??? but i am working upto 1500, but i may try it on wednesday). However, I am not sure exactly how i should train, that probably sounds a bit silly but should i work to a variable schedule like all drills one day, constant swim another, leg and arm floats??? etc??? I want to get the most out of every session, and i figure it should be like running i.e. short fast runs, long slow runs, hills etc... but how to apply that to swimming?????

Also I was interested in the Total Immersion method, does anyone have experience with this?

Any help would be great

Andy