Is off day trully off?

Do y'all do anything on off days? Or is a true off day? I need to work on core strength and don't know where to fit it in. I'm about to go into week 4 of 22 week triathlon training. I swim and run on Mondays- Wednesday I swim and  run. Tuesday I bike. Thursday off and Friday swim and Bike. Saturday run and Sunday bike.

Thanks,

Your rest days should be all rest, don't try to sneak anything in. You need to rest, not only for your physical self but mental as well. Do your core work on your bike only days. 

I better see you in March at Parris Island, you better not tell me you are too burnt out to race!!! You are doing awesome in your progress and following the plan like a champ!

I'll be there come Hell or high water. Thanks for keeping after me. You'll likely know before me when registration opens up let me know when you hear something.