I think you might be walking too fast if your legs and knees are hurting. You also might not be warming up enough either. 50 minutes is a long time to walk 7 days a week. Are you feeling pain while you are the treadmill?
I do a 5 minute warm-up then stop & stretch my lower body (calf raises & lower body stretches) and then do about 20-30 mins high intensity cardio then cool down for 5 mins. I then do about 20 minutes of stretching afterwards.
I don't think it matters how fast your treadmill says you are walking just as long as you are sweating and feel like your heartrate is up.
Also I just swing my arms,about waist height, I think you would get the same benefit from buying one of those waist things.
I hope this helps