Getting back into the swim of things- help

Ok, I swam since I was 11 year round until college. So its been 13 years since I have worked out with a team, I stopped for a lot of reason but I began to have supraspinatus tendon problems and Life took over. I am hearing new info on swimming --- what is TI techinque? I know I need to revamp my stoke style and have to move from a muscle through the water swimmer to the graceful stroke I have right after doing some stroke drills. How do I bridge the gap? I am just getting comfortable swimming sets of 100's--- 250s freak me out , not to mention swimming a 500 free straight. I get tired aerobically and I have a hard time determing if the tiredness ( weakness) is muscle fatigue or the tendon getting inflammed and signaling me to stop. I can swim 3,000 -3,200 in about 1 1/2 hours. I perfer to do other strokes, as they are easier for me to do. Like I would rather do a 400 im( ok so the fly is mainly 1- arm) with an extra 100 then a 500 free. However I know that endurance is a sign of physical fitness and want to be able to swimm a 500 free straight, flip turns and all. We just did 2 500's for time and although i did *** stoke i was able to swim it 7:49--2 min and14 sec off my best 500 time. What would be the best way to keep my technique up( ie longer stokes, increased kicking) and get to where swimming a 500 is a piece of cake? should I do more 100's with less rest to simulate a 500 or start doing 150's and 200s then 250's? PS- I am also about 100 lbs over weight--- my friend seems to think that I should use fins more to reduce muscle strain while increasing my heart rate until i lose more weight. any physical therapists/swimmers /coaches have any thoughts on that as well?