I feel for you. I am a SAHM myself and I have 7 children under my care everyday. I wouldn't trade it for the world! I had similar problems with my hubby, except instead of working out, my hubby's a computer junky. He works on the computer all day and then comes home and works/plays on the computer intil late at night. And to make matters worse, he had a 2 hour commute to and from work everyday. So we still never saw him. Well, we moved, and now we see him more, but he was still on the computer all the time. So, one night I went downstairs and asked if he was busy. And we talked.
I told him that there are a few times during the day that I find it very hard to handle all the responsibilities with the kids. For example, the transition time between the kids getting home from school and getting the 4 older ones started on their homework. Prepping and serving dinner, and then bedtime. These were times that I could really use his help. Even if all he did was keep our twin 2 yr olds entertained. He has started doing the laundry, since he's downstairs by the w/d. And he takes one of the 3 younger kids while he plays his computer. (He rotates who sits on his lap and he plays games that keep them entertained.)
So I guess that my advice to you is to ask your hubby for help. Perhapes to encourage him to teach some of the exersizes he knows to the kids. Make it fun. I know it seems silly, but kids LIKE silly. Encourage the kids to behave for daddy and that they can help you by letting you get dinner going or some homework done. Kids are naturally helpful.
As for meals, I take 2 nights off a week from dinner. My hubby cooks dinner one weekend night and the other night is usually leftover night. You raid the fridge or have a sandwich. It gives me a break from the stresses of the week and I use the time to relax a little.
Is your infant on a bottle or the breast? If he/she is on the bottle then you could ask him to get up on the weekends for the night feedings. But I'm sure that others have already suggested this.
One woman suggested taking a yoga class and I would also encourage you to take some time away from your family for you. You need the recharge time and yoga is a good way of getting in touch with yourself.
One other suggestion I have for you is to set up a routine that once hubby gets home from work, you leave the house to go to the library or some place that you can work on your homework without the constant interruptions. It allows you to get your 'work' done and forces your hubby to manage the kids. It can be for a couple of hours. And it doesn't have to be everyday, but enough so you can pass your classes in good standing.
You are an AMAZING woman and I really hope that things work out for you. You are braver than I to take on raising a family and school for yourself. Good luck!
I would also encourage you to have some Mommy/Daddy so you guys can connect as adults and have some stress-free talk time.