Avoiding burnout and having me-time absolutely helps you to be a better mom and wife. But you've heard the saying:
You can have it all, just not all at once.
That is VERY true. It's all about priorities. My girls are 11, 14 and 15. It really does seem like it happens overnight.
I'm thinking your priorities are (or you want them to be) something like -
1) You - your health, wellbeing and sanity
2) DS - emotional and health
3) DH - nurturing a good relationship with a lifelong friend and partner with whom you can enjoy life and support each other in your goals as individuals and a family
Then career, housework and errands fall somewhere under those 3 probably.
So, instead of taking time and energy away from one of your top 3 priorities, why not take it away from something else.
Could you do something like:
Sun - strength and yoga at the gym while dh is with ds
Mon - 4:30a run
Tue - 4:30a run
Wed - 4:30a run
Thur - 4:30a strength training at home
Fri - 4:30a run
Sat - 4:30a run
Then, ask hubby to help with errands, cleaning, etc on weekends so you can get things done more quickly and go out with hubby once a week. Seriously... make it a priority to spend alone time with your hubby at least once a week. Do something fun with him EVERY week!
A strong partnership with your husband, means having someone there who can help you do all of those mundane "have-to-do" things, so you both can get on to living fulfilling and meaningful lives.
Your statement "So DH gets an ENTIRE day to himself while DS is in preschool - I get nothing." worries me. Either hubby works from home and you're not giving him credit for that, or he's a slacker that not only allows you to work 50-60 hour weeks while he sits on his butt doing nothing, but also expects you to make your dinner and do the cleaning and errands during the few waking hours you are at home.
Either way, that situation is heading for divorce, which will really cramp your "me" time.
You absolutely have a right to me time. You should do everything you can do to avoid getting burned out. It's just good to try to do that in a way that doesn't take away from the people that you love and love you the most. If you love and support them, they are much more likely to be able to love and support you back, which will help you get the me-time, relaxation and de-stressing time everyone, including you, needs and deserves.
I hope you get what you need!
I'm off to read a book to my girls. Yes, they still like that stuff.