D.M.
I am a full-time working single Mom, and you know the extra income is helpful and it's not selfish. If you and your hubby have a game plan, than it's okay.
What I do is every other weekend, when I have my son for both days, I plan Adventures. I look up on-line and plan day trip or weekends away that are just for me and him. We've driven up the coast to Santa Barbara just to hang out at the beach, local zoo or park. I make sure it's just me and him, since he sees his Dad M-F for four hours (more awake time than I get during the week).
As for the weekdays, I plan my meals for five days while on my lunch break at work I look up recipes and then do my grocery shopping with my son on the weekend. He loves helping me with the list and cart...it's a total adventure for us and he loves helping. It's also a great way to learn colors and numbers! I make at least one day a crockpot day or I make extra and freeze for the next week. The night before I make sure I have everything prepped and ready so all I have to do is cook when I get home or reheat. Meals are always simple, something like rice/pasta, veggie and protein.
Since, I have every other Saturday without my son I do my errands then, but you could pick a Saturday or Sunday and let the boys have 'boy time'. This will give you a little time to run errands and them, time to bond and have fun doing boy stuff.
It's tough to transition, and yes the guilt is awful...some days are worse than others. But, I know that this is the best thing I can do for my son's future. He gets to spend time with his Dad and my Mom and goes to preschool two days a week. Just remember this is not selfish, or silly, it's what you have to do for the good of your family.
Sit down with your husband and work out a game plan for errands, cooking and cleaning. Make sure you share in the duties for the house and work as a team. It will be okay!!
Good Luck.