Eventually though you'd be running at full speed and the extra feet beyond what it took to reach that point would mean nothing as far as damage goes. How long does it take to get up to full speed at a run. Probably only 20 or 30 feet.
I just pulled a number out of my bag of holding.
But you'd think that it would be akin to a bull rush (at least ten feet) and the damage is based on 'how fast' (running x3 or x4 normal movement?)
id say x3 would be d4s since its less over all speed
but for x4 id have to say a d6
of course.... if we really wanted to get into this, use falling damage as a base.
An object falls at (9.80665 m·s−2 [approx. 32.174 ft·s−2]) which would mean that a character would have to be moving roughly 180 per round. So we're looking now at a monk. For every ten feet at 180 feet per round (round 32.174 to 30 feet, multiply by 6 [6 seconds per round]) to take 1d6 per ten feet charged.
So when adjusting I have no clue what to do. I do; however, believe those numbers for ft per second are accurate.
we would also need to take weight into acount, of course, this would be charging full speed into the wall...which I (as DM) would have you take 1d4 damage and make a fortitude save (DC bleh) or be stunned. Since...if youve ever run into a wall, you're a little shaken up after.