How do I achieve the 1k in 15 min? How to prepare for 4k OW?

Hey experienced swimmers, really need your help; 40 year old dude here who only recently returned to swimming. Currently swim 100m in 1minute 21, 1k in under 17 minutes and 2k in about 34 minutes in a 25m pool without pushing myself too hard... Goal is 1k in 15 min in the shortish term - and I want to do a 4k OW swim in 11 months' time. Major restrictions: No Masters, squads, coaching or even lane ropes where I live at the moment (Bavarian village), and only can train 2-3 times a week for about 1.5 hrs in a 25m pool (though there is a 50m one outdoors in summer.) Have kickboard and paddles for drills but nothing else. Thanks in advance - all and any advice on training, plans, tools, drills and even nutrition appreciated!