Join Date: Mar 2003
Location: London, England
How long have you been cycling? The reason I ask is that when I first started riding I used to get a dull ache in my lower back, usually after 20 miles. I thought, like you, that it could of been bad posture, but it just turned out I needed more saddle time. Don't get me wrong, it may be that you have to change the set-up of your bike, like MichaelW suggests, its just if your new to cycling then it maybe that your body just needs to adjust to the activity.
Anyway I hope you work it out, Portent.