I have flat bars on my bike and I don't think they are inherently bad. I can't recall any major pains in the hand/wrist area, but on a couple of occasions (after several days of touring in flat landscapes) my shoulders have complained a bit. I think the best advice I can give is to relax your arms and hands, and don't use the most obvious grip that much (my preferred "distance" position is resting the hands on top of the break handles, which happens to be a very similar position to the hoods on a drop bar). This season I have done three ~300k rides on my flat bar bike, one of which at the end of a 700k three day tour, without any discomfort in my arms or hands so a flat bar bike can definitely be made to fit. It also takes time to condition the body to new limits, and if you are comfortable riding 40-50 miles then I'd say that the pain should diminish after a few more 70 mile rides.

/Csson