1. Near-certain injury. You cannot expect your body to perform to such standards in its deconditioned state. Your body needs time to build up strength and endurance. Begin with short bouts of exercise a couple of days per week, and progress from there. It's tempting to rush the process, but not wise to do so.
2. Complete failure and an abandonment of your routine altogether. The mind can only process so much change so fast. Adopt small changes and just one or two at a time. This is true for activity and diet as well. Make sure you have made the new changes habit before introducing new changes. If you set unrealistic goals, your body and mind will be sure to show you how unrealistic they really are, leaving you only with feelings of demoralization...and a pair of lightly used sneakers sitting in the back of the closet.
As always, be sure to consult with your health care provider before starting any fitness routine. He or she can recommend any restrictions you should follow based on your medical history and conditions. If you need additional help getting started with an exercise program, a fitness professional can design and help you safely implement a goal-based training program.