If you can comfortably swim the distance within the cutoff, you could most likely finish a Half Ironman tomorrow (as mentioned by plant_based). It would probably mean walking the majority of the run leg, you'd get saddles sores from the bike and generally not enjoy the event at all.
Setting an overly ambitious target after a break can often lead to injury because you feel the pressure to get in the mileage. I would target Sprint Distance triathlons. They will give you a good idea of where your current speed and endurance sits. From there you can choose when to progress to Olympic and then Half Ironman.
Great point and you have illuminated to me why so many times I've injured myself "on the comeback"