Mugulian,
Welcome and congrats on your progress so far! You're doing great. Like everyone else, I would not be concerned about speed at all. I run very slow too but I learned not to be so concerned about speed when I'm trying to increase my distance. I will do speedwork when I'm ready to improve that area of running. You will not be in the last of the pack, rest assured...but even if you are..is there anything wrong with that?
Secondly, i wanted to tell you that my running partner has RA too, and she's only in her early 20's. She was diagnosed when she was 13. She has run 3 marathons, and getting ready to run her 4th with me in a couple of weeks (my first). She has managed to do it successfully, AND she's WAY faster than me! However, with that said, she has had her fair share of difficulties...just make sure you keep in touch with your Rheumatologist. Your joints won't recover like those of us who are disease free, so you may need some extra support. I see no reason why you can't accomplish your goals, just be smart and methodical about it.
Good luck and let us know how you do...you can stop lurking now and join us!!