It's Harder Than You Think

I'm still finding it surprisingly difficult to do that which I know I must - namely to sit down at my computer and just write.
I just can't seem to take that first step to getting words down on paper. I can do all the planning, worldbuilding and character building stuff - have a mountain of it in fact - but when I sit down to write what I've planned, nothing comes.
Take, for example, that fantasy story I told you about. I have a great world to set it in, a main character I really like, and I have a good idea of where the plot is going, but I can't write it.
It starts with my MC, Cornelius, at university, learning to be a mage. I've been to university, I didn't study magic, but I still have a good idea of what it's like to be a student. All it should take is a little imagination to create the feel of being a student in magic. I don't know where to start though.
It's the same with my martial arts guy, Ryan Philips. I'm less certain where the overall story is going in his case, but I have a very good idea of his situation at the start and still nothing comes.
Writer's block they call, but for me it's not just a block, it's a bloody great stone wall, surrounded by a moat with crocodiles and sharks swimming in it.
It's odd, because I get the compulsion to write at the most inconvenient times. I'll be lying in bed, trying to sleep and suddenly feel this overwhelming urge to write and then have to weigh up the pros and cons of sitting up writing or sleeping to enable me to get up for work in the morning. Or I'll be sitting at my desk at work (like I am now) and be so bored of what I'm doing that my characters start coming to life in my head and all but chant 'write me, write me, write me'.
I've made the oath "I will sit down and write tonight" more times than I can count, and have even kept to it for a couple of nights. I suppose what I need is to stop whinging and just get on with it, but it really is harder than you might think. For all the times I've sat down and managed a couple of hours writing, there's been two where I've sat down and achieved nothing.
Ah well - such is life. One thing I do know is that I won't be giving up soon - it's just not an option. I can't get these stories or characters out of my head. One day I will, they'll be out of my head and down on paper. One day.

Okay, rant over...carry on with whatever you were doing.