Back in the pool after several year break

Sorry if this has been posted before, new to the site and still learning to navigate. I swam competitively in high school. That was 10 years ago. Have been in the pool on and off, but nothing steady. Recommendations on how to ease back into shape? Should I focus on distance workouts? I am just looking to get in some cardio, not interested in competing or getting faster. I am shocked to find myself winded after more than a 50/ 75. I can do a pretty easy 200 when I get in. If I take a 5-10 second rest after each 50 I can swim a strong 50, have great technique and feel like I could swim for hours - however when I try to swim a 100 in the middle of my workout I find myself getting fatigued quickly. Recs on how to structure workouts after being out of the pool for several years - obviously I'm very out of shape :/ Thanks!