Let me get this straight, you have four boys, four, and you can't get any exercise? How is that possible? I have 3 girls and spend at least an hour everyday just cleaning up after them. It's not uncommon to find me wearing wrist/ankle weights and headphones, shaking my booty while doing all that blinking housework my kids left for me. I have a friend that gets her workout by imitating her kids, 15 minutes/kid x 4 kids = 1 hour. Her kids LOVE it, they call it "monkey mommy time", but hers are all pretty little. How about taking the kids to the park with a soccer ball and boot it around, heck pushing kids on the swing is a workout, or to the beach, skate park, swimming pool, believe there is a lot of cheap/free activity to be had out there. We actually walk all over the place, walking is really good exercise, not just for you but for the kiddies.
From my own experience you can't really get in shape with a single 30-minute workout/week. I used to go twice a week for an hour and never gained any ground on getting in shape, I had to up it to 60 minutes/day 6-7 days/week. My grandmother has a stand for her bike, that turns it into a stationary bike, she rides it all winter in her guest bedroom. Do you have any place where you can set your bike up and get that 30 minute ride in more often? If you live on the 3rd floor, you could run sprints up and down the steps. You might also post a sign in the complex, maybe another lady has the same issue and you could swap kids and each get in a workout at the gym. Just a thought. Good luck, I completely understand how hard it is to workout with children hanging off your legs.
Starting weight: 244.6 lbs. 10/01/09
Pounds lost: 80.6 lbs
Current weight: 164 lbs
Goal weight: 120 lbs
Weight to go: 44 lbs
Goal Date: 1/1/13