Re: ~ 100 oz. of soda per day: Long-term effects?
100oz of soda is a lot of sugar -- probably about 280g or 1120 calories per day. (And most soda sugar is really high fructose corn syrup, which some think is even worse for you than regular sugar when consumed in large amounts.)
Since you said you do not do much exercise except when training for fitness tests, it is likely that your metabolism will go down due to muscle loss. 1120 sugar calories per day from liquids is likely to make you fat, increasing the likelihood of diabetes and other problems aggravated by body fatness. And if you are already diabetic or pre-diabetic, drinking that much sugar calories is to be avoided.
You are lucky that all that soda has not caused more cavities in your teeth.
Even if you limit other food consumption to keep your calories in balance, having half of your calories come from sugar with no other nutrients makes getting a nutritious diet very difficult.
Diet sodas are not that great either; some studies indicate that their sweetness confuse your body into thinking that calories are coming and that when the calories do not come, you may eat more and get fat.