IM training - Extra hours go to Strength/flexibility training OR more swim/bike/run

My general philosophy is more strength training during the "off season" and focus on sport specific training during race specific builds.  So, if your body can handle more training stress, and still recover, during your training for IMMOO, spend it s/b/r.  For most people, that's going to be best spent on the bike, unless they're an especially week swimmer.

The exception to the above is someone who has a specific injury history or movement limitation that is best addressed through strength training.

A third option that's best for some people is to put the available time into sleeping.  The average AGer is more likely to struggle with recovery during IM training than they are likely to simply have extra time available for training.  Don't forget that improving fitness requires both training stress and recovery from that training.

In addition to sleep, how about: read a book, watch TV, spend time with friends, remember your kids and significant other exist, walk the dog, sit in the park and people watch.  You know, normal life things also known as "recovery time".