No problems with downhillers and no doubt a full day of it will batter your muscles as much as an average xc ride (all be it in a different way), but I do love uphill! More or a challenge!
Rode the Brecon Beast in Sept and it does the Gap road backwards, and I was very surprised to see that 95% of people that I saw on that section pushed the whole way, didn't even attempt it, and only one other person that attempted to ride the whole thing! Granted, I felt like I was going to die at the top, and there was still 40km to go, but it's the achievement… or is it just me? Fair enough if you've got a massive DH monster, but that wasn't the case there.