I'm training for a marathon and am having pain in my plantaris muscle, how can I help myself?

Get evaluated. The plantaris muscle's location allows it to interact with the plantar fascia as well as the achilles tendon. Consider seeing a podiatrist for a foot/ankle check, gait evaluation and shoe examination. Depending on the findings, some reasons for the plantaris pain may be due to abnormal pull due to poor biomechanics. Treatment may include orthotics, massage, physical therapy or gait training.
Plantar fascitis. The most common cause is too fast training, and shoe with no arch support. Second cause low thyroid (tsh needs to be less than 2.0. Your uric acid may be high needs to be less than 6.0. Hba1c needs to blow 5.7.
See podiatrist. You will need rest and medical attention to correctly diagnose and treat the problem. May have to pick another marathon to allow recovery time. You CANNOT train through this one. Been doing marathons for 35 years and had plantar fasciitis once. Had to step back and readjust the marathon date and allow time to heal. Be smart and fix it first. Plenty of marathons left. Good luck and good sense.