NGC4889, these are may opinions, others may see it differently.
1. Swimming in a 13m pool won't hurt, but it may slow development a bit especially as you become a better swimmer, and I would imagine it could get boring then too.
2. If you feel like you are improving with your current regimen, keep it up until you want to try some intervals and more challenging practices. Here is the link to the USMS website with beginner swimmer workouts or just go to USMS home>Forums>Workouts>Basic Training for New Swimmer: http://forums.usms.org/forumdisplay....by-Wendy-Neely. I'm sure Wendy Neely would be willing to help and answer any questions on that forum. And there are many other type workouts posted each week for all sorts of different training objectives. Some are probably to advanced and won't work in a 13m pool, but look through them all and figure out what works for you. If nothing else it will give you a sense of others are doing.
3. Swimming requires coordinated effort by many muscles throughout the body and significant conditioning too. It's more complex than jumping on a bike or running around a track in my opinion. But, you don't have to become Michael Phelps to become a reasonably good swimmer, even as an adult this can be accomplished in a relatively short period of time. Most good swimmers are very efficient and that comes from practice and from use of proper mechanics. Without proper mechanics you waste energy very quickly and that is probably why you are tired. Using some "pool toys" - fins, pull buoy and paddles may help with the mechanics, as well as using the right drills. But bottom line, it is very difficult to teach yourself to swim well and you should try to get to some coached workouts and/or find a good coach who can spend 1/2-1 hour periodically teaching you what changes to make with your mechanics - this is a great investment and will speed up your progress. You may be practicing and ingraining bad habits that you will have to undo. Make sure your diet after workouts is well thought thru so that you recover and gain from the workout - most swimmers are pretty hungry after workouts and not all swimmers are lean. Your body needs fuel for the workout and the right nutrients afterwards to make the muscular adaptations. There are many good articles on training diet on the USMS and USA Triathlon websites.
4. There are lots of swimmers using dry lands and weights as part of their weekly workouts. For me dry land twice a week (if that) seems plenty. Some of the sprinters and younger swimmers may doing much more. Ian McLeods book Swimming Anatomy is a good source of information and provides dry land training techniques for all of the muscle groups and David Salo's book Complete Conditioning for Swimming provides guidance on conditioning. They are both available in paperback from Amazon.
Hope this helps.