Shinsplints.....

check out http://www.beginnertriathlete.com/shin_splints.htm if u haven't.  look at your past and current training.  r u going up on miles too fast.  general rule of thumb is not to increase mileage by more than 10% a week.  always have a ez week once a month.  check your shoes.  if not already, go to a professional running store and get fitted.  THE BEST thing u can do for yourself.

if u want, outline your training for the last month and we will take a look at it and offer some more suggestions. 

for now, ease back or bike and swim till u r pain free.

i have flat feet too and have (had???) many problems with them, when i went to a runners supply store i learned that they offer 3 basic shoe types...cushion, stability and...something else..support?.  anyways, i wear some good arches (spenco) for my work shoes BUT the dude (cross-country coach) said that FOR RUNNING u dont' want any noticeable arch in your running shoes as when u run, your compressing that whole area and all the force would be on your arch leading to future problems.  i used to wear arches in some reebocks but my feet felt horrible.  i was able to get properly fitted in a running shoe that offered me the support i need without any xtra supports in the shoe. - asics 1050...its in the product review.  my tendinitis is gone thanks to a properly fitted shoe.

of course your feet could be different than mine and some people do require a little extra something-something in their shoes.  go back to your running store and see what they say...may help alleviate those shinsplints...which is something u shouldn't work through.  if u don't have a dedicated runners store near you, let us know whereabouts u live and we will find one.

OH YEAH..DONT PENNY-PINCH ON YOUR SHOES...IF YOUR DOING A HALF IM...YOUR SHOES WILL BE THE MOST IMPORTANT PEICE OF EQUIPMENT...NOT SOUP-ING UP YOUR BIKE.