Are skimpy Speedos acceptable in public pools? And should one do anything to make oneself appear more manly downstairs?
Skimpy trunks are much better for swimming in, if you're going to do any distance. I took the plunge (!) and got some, and the difference compared to baggy shorts was instantly noticable; 1-2 seconds a length faster due to far less drag. Lots more comfy over longer distances too. Don't worry about yer cobbly bobblers; immersion in cool water will lead to even less drag....
Join a technique class, take it easy and enjoy it. That 4.1k training plan is ridiculous; will take years of dedication and regular training to be able to swim that fast. I can do 1 mile/1600m in 30 mins, and that will knacker me. Haven't bin for a couple of months due to illness/demotivation, but I was going regularly 2-3 times a week, bin to a 12-week technique class, so not a bad little swimmer. Some folk take it far too seriously. Swim for enjoyment and fitness.
The difference it will make to your physique (unless you are already Well Buff like me) will be very noticable after just a couple of months. Really builds up your shoulders! Helps with flexibility and overall good health.
30-40 mins 3-4 times a week is excellent. Go for the 15-20 mins to start with, no shame in taking it easy at first, then build up gradually. Set yourself little targets; fastest 100, fastest 500, fastest 1k etc. Mixing it up during the week is good; one session of doing short sprints, then next time do a long slow swim, sprint then slow, etc.
But yeah, enjoy it, that's the key thing.
Would I be right in assuming there might be lots of healthy senoritas at your local pool?