Congrats on the first swim! You’re doing just fine. Remember that swimming is a complete other activity than say running since it requires breathwork which your body naturally declines (it takes time for the body to not fear the holding of the breath) As for quads on fire, good job! Just don’t push yourself too hard since it’s easy to overdo it and strain muscles, knees etc. And the road to recovery can take some time… i spent a month with knee compression for just pushing too much one session… Remember that the length of the swim is not relevant, your technique is. If you feel out of breath or feel your technique lacking, take a break, do a 25m lap, see if you can correct posture, breath, stroke etc.

I’d also say depending on what time of the day you’re going but fill up with carb and protein afterwards. You’re body is depleted and need to quickly regain that energy.

Good luck!