Push all of those excuses to the back of your mind, to the bottom of your stomach, down to the ground and keep it there! If people look at you, who cares? If you're not flexible, so what?
Use the class, the three days, the week, and (if you're lucky) the month (!) to kick yourself into gear and create better habits. Make sure you ask questions and accept all the help that is offered to you. Try a ton of gyms in the span of three months if you have to, but do it. You'll find a teacher that you like, a new format that you love and hopefully start a trend in your life that you won't want to lose.
The only thing that you should make sure you do, is consult your doctor, listen to the instructor and don't try to compare yourself to anybody else in those locations. Comparison can boost your ego, or break it. Boosting your ego in this way is hurtful to the other person, and if your ego gets insulted, you might not continue the original good habits. Injury can happen when you over work yourself, as well. So, take it easy and go at a good pace that's gentle with where you are in life.
I personally joined yogaglo.com recently. I love the teachers there. I was offered a 15 day free trial, and I am going to keep their services. They have 30 minute sessions that, after completing, feel fantastic! It's better then doing nothing, and I don't even have to leave the house! I also have a gym membership, but due to an injury I abandoned the gym for three months and now my toddler will not be left at the baby center without a huge melt down. That is not an excuse to cease attendance, but for now, it's keeping me from the gym.
Yes, there is a potential that you will buy into the membership, but is that bad? YES! It's bad if you never go. You are literally just throwing money down the fitness-center drain. Don't buy new gym clothes (maybe a new pair of shoes if you're running), don't buy "cool accessories" or a new yoga mat, just go in your pj's if you have to, but go! Do it for your health, and your sanity. I hope I don't have to list the tons of empirical evidence that support exercise (in any form) as beneficial. But if so just: go here.
If you care about your well being, be an opportunist and use the free trials at all the fitness centers near you! Good luck, and may you find the fitness center that keeps you feeling fit and healthy.