400m sounds great. SSI's is 800 meters, but with snorkel, fins and mask (I think PADI's is a straight swim). I have a shoulder problem where I have to work to get my right arm above my shoulder- can get to valves, etc., no problem but putting repetitive force on it is painful. So with my legs and large stiff fins, I was able to do it in the maximum 20 minutes without using my arms, and was not far off the preferred 16 minutes. Now for our 200m tow, I had the privilege of towing someone who is 300#+. I did it in the preferred time, but wow, what a drag... ;-) and was I breathing hard.
Good luck. Many of us dread it, but most of us succeed, as you will.
Terry