Congrats on the new job!! I agree with Darkrunner. It's not weakness at all, quite the opposite in fact, takes strength, insight, and wisdom to know what is working and what is not. It is a matter of taking care of yourself and doing what feels right to you. It is great you have gotten into this new position and like what you are doing; overdoing it is to your detriment. You may find later you want to take on more once you've adjusted but it is a big change to jump into 40 hours. I commend you for wanting to make sure there is balance and to not get overwhelmed. They will find someone to pick up the additional work and to fill the vacant position, no worries there. Focus on what is best for you to maintain your balance.