I can't stress enough how much you will appreciate it if you do ALL challenges before moving on to the next area! It really isn't fun doing 189 challenges when you have everything else completed, I found out the hard way.
If you are struggling on the Ranier Peak blitz, just make sure you take every single shortcut and use the Valkyrie bike. Keep your cool and make sure you know where you are going! If you crash once then restart, because you won't make it otherwise. You CANNOT finish this race by simply following the track.