When/how to transition from "lap" swimming to workouts?

Started swimming again this year in an effort to get in better shape. Haven't done it since college (I'm 43), and that was just for exercise. Did it as a kid, now my kids also swim. At any rate, I'm just doing laps right now. Usually 6 sets of 500, occasionally only 5 and occasionally 7 or 8. ~5 days a week when life doesn't get in the way (so about 15,000 yards per week). About 25% ***, 25% back, and 50% free. I have occasionally thrown some fly in there, but I just can't do it without it fatiguing me to the point where the remainder of the set is very suboptimal. At any rate, I am wondering at what point I should start looking to transition to doing workouts, rather than just laps. There is no Masters program that will work for me, so I'd just be following the posted workouts from the forums. Should I just jump right in? Should I start from "week 1," or just hit it right in the middle? Also, should I just be looking mostly at the general workouts, of mixing it up with the IM workouts to get more strokes, or start with general and slowly work my way towards the IM stuff? Anything y'all need to know to help guide me the right way?