Recovering from Stress Fracture

Hi yall! So I'm recovering from both a concussion I had at the end of March (cleared in the second week of April) and also a stress fracture in my L5. I used to run a lot but i had to take off 2 weeks when I got my concussion and then I could only run short distances for 3 weeks because of my back. I'm trying to get back my endurance I had before all of these injuries as well as get back my abs. The problem is I lost all my stamina and I am really struggling. I lost some muscle mass and so I feel like I'm getting fat. I really need help/advice on how to deal with this and get back into shape!