Running a 5km with fibromyalgia

I want to run my second 5k (the first was waaaay too much mud and 7 years ago). I have fibro and rheumatoid arthritis so I want to be reasonable with my expectations. I don’t think I will get to the point of running the entire time but I was thinking of doing intervals with running and walking. Right now I can do 3,5km with 1 minute running and 2 minute walking intervals. I am slow but I really enjoy the calmness it has brought to my mind. I was thinking of aiming for a 40 minute 5k so what would be a good interval to plan for? The race is on March 16. Any suggestions are appreciated