Female | 70 years old
Type 1 diabetes: Insulin deficiency CAN happen in adults.
The natural progression of most diabetics is that they have less insulin-making capacity. Thus, using medicines to make you make more insulin are not effective. This would be the sitagliptin (Januvia).
Often is the case that people need insulin because their pancreas simply is not able to make enough insulin to keep up with the sugar intake. It is not your fault. Genetics play a huge role. There are other medications out there to help that work in a different manner. New medicines make us urinate out excess sugar, thus decreasing blood sugars and help diabetes. In the end, insulin replacement is best (and safe if dosed correctly).
I am proud you're exercising daily! I am sure the sugars would be higher if you didn't do this!