Hi, thank you for providing the brief history of you. As you mentioned you have pain due to plantar fasciitis, and also you have undergoing 8 weeks of physical therapy. Also, you are doing some specific exercises and stretch on most days it is good to hear you have been into physical therapy. Also, if you are planning to run a marathon, your calf muscles and the tendoachilles muscle needs to be trained in specific on both the sides to perform the balanced GAIT cycle and also the biomechanics of the muscle has to be made to the optimum to avoid the pain during the marathon. For which you should be next put into the sports retraining program to get the optimum results. Understanding the videographic representation of your gait cycle while performing normal walk, jog and running is recommended. When the soccer player , cricket player, long distance runner etc can get help for the better performance in sports i am sure even you can get help and you can win the marathon with a specific sports retraining. I have been dealing in my clinical practice with sports injuries, but dealing this case as a special case to achieve the optimum results the rehabilitation plan is totally different. A conventional stretching and strengthening is applied initially and a specific program is implemented to get the optimum results. Also, my clinical experience has 99% of the success on the sports injury cases with specific and optimum training , i am sure even you can get better. I will advice you to find a sports physical therapist and get rehabilitation program designed. Regards Jay Indravadan Patel