You are only counting half of the picture men that where commanders pre and early war probably already had at least 6 or 7 years in subs at sea and some a few years in surface ships as well so an early war sub commander easily had almost a decade of sea time under his belt or more. Even guys that became COs later in the war likely had at least a few years in subs as lower officers prior.The US Navy preferred sub COs to have at least a few years experience prior to becoming a CO.
Here is a listing of the actual lengths of his war patrols or at least some of them If you really wanted to you could request to see his military records and find out exactly how many sea hours he had. Sounds to me like he was one of the most experienced sub officers actually still at the front and not at a desk.
http://www.uscusers.com/balsam/196.htm
That is 10 war patrols over a three year time span not 3 years actually at sea.The 10 war patrols might have equated to a full year combined I dont know you would have to look up a given CO discover his entire sea time in subs if it is listed or calculate the total time at sea for each patrol he went on as a submariner. I think they measure it in hours and they calculate total miles covered some long term sailors are given an award for huge numbers of hours.