I completed a very hilly race on Sunday and found the downhills tough. Its a different kind of effort though.... uphill its burning calfs and an oxygen debt and downhill it seems to be the impact of striding and trying to stretch out.
I'm with Dave on this, I find that after a downhill my shins are really hurting and I'm ticked at having to actually slow myself down. - I have never had to slow myself down going the other way.