As I understand it, there are two ways that vigorous daily exercise helps manage ADHD. (Note I am not a medical professional but have done a fair bit of research and reading w/r/t addiction, depression, and ADHD)

#1- in the short term, exercise provides a prolonged dopamine/endorphin release which just "feels good" and helps calm the restlessness and irritability that plague ADHDers

#2 - in the longer term, exercise helps to beef up the dopaminergenic pathways (just like your thighs!). This combats one of the neurobiologic roots of ADHD, that the physical structures that generate, transmit, and absorb DA (dopamine) in the brain are less developed than those of non-affected people. So it's very beneficial to improve DA 'fitness' because it makes each instance of #1 work better, in addition to helping baseline (non-exercising) concentration.

Some good links:
http://www.mastersofpediatrics.com/c...lecture5_1.asp

and everything off:
http://info.nature.com/cgi-bin24/DM/...BWgI0Cv0oeT0Ez