Is running not in the cards for me???

Hi there! I am a 5'7, 248 lb woman. Last July, I had weight loss surgery and to date have lost about 60 lbs. I have always wanted to run, but because of my obesity, I couldn't. Last April, I attempted my first 5K at 306lbs. It took me an 1.5 hrs, but I finished! I have been attempting the C25K since the beginning of the year, but haven't been able to finish. I had to have knee surgery to repair my ACL in 2003. My doctor told me that arthritis is inevitable and will show up by the time I'm 40. Well, I'm 39 and it showed up about 10 years ago. I lost 112 lbs in 2001 and though my knee pain was better it never went away. I gained that weight+ back. Now, my knee pain is excruitating! The doc has given me Zipzor and Volteran gel for the pain and says that a knee replacement is inevitable, but not at my current age. Everytime my knee feels strong, I attempt to run, and BOOM I'm hurting again. Should I just give him the dream of marathons??