In reply to Craftycarper:
A huge part of getting hillfit is to remember that you love being in the hills. Many years ago I used to be miserable once I got tired and wonder what the hell I was doing there. Eventually I sussed myself out and was able to divorce how my body was feeling from the enjoyment I got out of being there. I still get tired some days and for parts of most days, but honestly enjoy ALL of every day - and just the fact of that enjoyment can make a huge difference to your performance.
So if you don't genuinely enjoy hill walking and being in the hills, forget it and think about taking out a gym membership (or some other sport), but if you do just remind yourself of that when you get tired. Sounds very aerie faerie, but it worked for me.
Oh, and to return a little ukc cynicism to this gushing nonsense... it's Snowdon, not Snowden.