I've been taking methadone for about 20 years, and constipation was never a problem, until I got more sedentary.
You'll do better if you walk!! (You'll do better in every way, you'll feel better too).

But, I found that Miralax (over the counter now) is a lifesaver for the constipation. I get by on half a dose per day. It's a very gentle powder that you can mix with anything. Completely tasteless.
I take 50/60 mg of methadone/day. (It's been so long, I don't remember how long it took to build up, but just a few days... I think.)

I also keep on hand these 'fleet enema things' they're very small, and give relief in about 5 minutes. (sometimes I just forget the miralax).

Maybe there's another med that you're taking that's helping to cause the problem, but you've got to walk around a couple blocks per day, exercise is very important to us with rsd!

Best of luck!

pete