For anyone who hasn't rolled on concrete yet, let me assure you, concrete hurts worse than grass, you will probably get quite a few bumps and bruises while you perfect rolling on differant terrain(gravel being pure murder) I got most of mine in while training in Akido, rolling is a major factor and you want to be sure you can do it at any moment.
It's really not that bad, if you can roll on grass from a fast run or from a medium landing without getting hurt, then just do the same thing on the concrete, it may hurt at first, but the pain makes you tough, and soon you wont even notice it. I am a skate board fool, I love manuals but I bail ALOT. Hell I kinda like the pain, it inspires you to do it right.
Just my thoughts. Don't EVER (I MEAN NEVER) hesitate or doubt your ability to do it, thats what keeps you from being fluid and relaxed.