Knee pain in both knees

Last yr while running, a sharp pain went right through my right knee and I could not even walk on it afterwards. Doc said it was tendonitis but he never actually looked at my knee. Over the year the pain went away but occasionally I would feel “pressure” on my knee while running. Last week while running (training for marathon)I wore a knee strap bc the pain started up a bit and 12 miles later, my left knee felt a sharp pain (not bad like last yr) and when I took the band off my right knee when I got home (after 15 miles) something hard was sticking out of the lower right hand side of my knee, causing a lot of pain. Does anyone know what this is? Is it tendonitis or something else? If I continue to train for the Marathon in October, will I completely destroy my knees?
Thanks!