Losing weight & getting faster?

Hi, I'm new to the forum and this is my first posting. I swim, both for the love of it, but principally as an aid for weight loss. I'm a 53-year-old male, 5' 11" tall, otherwise in good health but rather heavy. At present I'm 246lb. I never swam competitively in the past. I've been losing weight at around 2lb-per-week for the last 17 weeks, with the odd blip, using the 5:2 diet. I swim in a 25 metre public pool three-times-a-week, for a minimum of 2 hours each session. I've lost 21lb in those 17 weeks and reckon I'll be able to continue at the same rate for at least another 36lb. After that I suspect the low-hanging fruit will have gone and it will be a fight to get the remaining pounds off. The target weight is 189lb, and I aim to hit that by Q3 2017. At present I'm 'clinically obese' having been 'morbidly obese' in the past, although as 'morbidly obese' I could a run 've switched to that stroke only in the last three years. Before I only swam breaststroke and two lengths of front-crawl would have left me with tunnel vision (so I didn't do it!) Swimming initially with my son I made an effort to switch to the front-crawl and after six months I could finally swim a mile (64 lengths) with no difficulty, though not necessarily all-that-fast. Since then the speed has improved, but only recently have I seen a step-change, coinciding with a relatively small weight loss. I don't dive-in and don't do any flip turns (my substantial buoyancy tends to make these difficult!) I use a Swimovate Pool Mate Live Swim Watch and I have a Finis TT Pro. I'm not a club member and generally swim alone. My swimming technique follows TI, but I've not received any formal coaching. As my weight has reduced recently, my PB's have improved, some markedly in just the last month-and-a-half. 50m 0.43 (this hasn't improved for a while) 100m 1:27 (previously 1:34) 200m 3.20 (previously 3.25) 400m 6.58 (previously 7.18) 800m 14:43 (previously 15:05) 1500m 28:49 (previously 30:01) As the weight drops-off I'm finding that my general speed is improving. For instance a month-and-half-ago most of my 200m swims were 3:40-3:45, even though my PB was 3:25 (set in June 2015). Now all my 200 metres are sub-3:30's and the Xmas target is 3:15. I rarely swam a sub-7:40 400m but in the last fortnight every single one has been sub-7:15 and many 7:01-7:02. So things have improved and improved consistently. You might note that I swim the 1500m quicker than the 800m, but that's likely to be addressed in the next few weeks! With all of the above, I have some queries. * For the specific purpose of losing weight, what swimming regime should I follow? Sprints (50m-100m) or longer-distances? I swim 'sets' but I will reach a physical limit at my current weight a little quicker than I would if I was lighter. At present 4 x 200m all @'t think I'm going to be able to perform a reliable turn until I get down to at least 224lb. I've read various estimates; such as that losing 14lb will contribute 5 secs off every 100m, but I suspect that say between say 224lb and 210lb the difference will be marginal. Someone who has lost a substantial amount of weight might be able to answer this. Any advice will be gratefully received!