I would suggest that you get a properly sized used hybrid to start with. You could start with a road bike, but if you are really overweight, it will probably fall apart very quickly. Besides, people with fat @$$es generally don't like the way a road saddle feels, at least not at first. Loose the pounds first, then upgrade to a good road bike.
Expecting that you will be able to go from a sedentary life to doing marathon road racing in a matter of merely months is extremely ambitious, to say the least. Try not to be unrealistic.