In other words: Quality over Quantity
I hope you spend your time wisely this off-season. Years ago, I was of the mindset that more is better - running longer distances, going to the gym for 2-plus hours, and the dreaded "no days off" mentality. My wiser self would have told my younger self to be smarter with the way I trained.
I can't guarantee that, for all pitchers, that "less is more" but do know that "too much is not better". Give your workouts the most focused effort you can muster. If you need earbuds to block out distractions, fine. If you can get yourself in the workout "zone" mentally without any extra help (like earbuds), that should tell you that you have the capability of blocking out the opposing dugout and crowd in pressure situations - think about it!
Your workout time is valuable! Challenge yourself to be efficient with your movements - think about balance, keeping a relaxed flow, and well-timed explosion. If you aren't lifting as much weight as your teammates or friends, do not be concerned! Do what your body will let you do, no more and certainly no less! Be working on balance, flow and explosion regularly.
Avoid going through the motions. Being in a gym doesn't give you automatic fitness gains. Get in there and give it the effort you deserve, don't try to fool yourself and expect that you will get great results because you are at the gym. Get in and out of there with your best effort of that day.
One final thought - remember to keep throwing a ball during the off-season. It will help maintain tone in the smaller muscles, tendons, and ligaments surrounding the joints. This will be a great way to keep your arm in tune as you build a stronger body. There is an injury risk from getting stronger, thinking you can let it fly immediately in the spring, and finding that you have strained something in the arm or shoulder. It happens more than one might think. Stay away from an early season arm issue by keeping the throwing motion going now.
As you get into December and January, introduce your body to some quality training. One or two days a week will make a difference! Work with a goal and a purpose, and enjoy the results next season!
If you give complete focus in your workout, you will see progress, guaranteed!