Most anyone here is going to advise against a walmart bike. They are of low quality and often assembled poorly. I had a walmart bike a few years ago and it didn't last two months before it fell apart.

If you have to get a walmart bike, then go for the singlespeed or fixed gear as there is less to go wrong with it. Walmart has a mongoose fixed gear that looks ok to me.

You should take the walmart bike to a real bike shop to get it checked out too.

Overall I think you would be better off going used off of craigslist. Good luck with your weight loss.