In reply to alps_p:
You desperately need to get as much experience as possible with altitude if you're planning on going high. My body reacts a couple different ways at altitude and I can now tell fairly early how it will go. Though I've only been up to 4200m.
Its a kind of exponential curve with altitude, 100m of gain at 3000m feels like 10m of gain at 4000m (or something) so if you can do 1000m gain at a run at sea level you might still be crawling 1m at a time at 5000m.
Sorry to be so pessimistic. Learning to suffer is the best training you can give yourself at sea level to train for altitude.
On the other hand some days I've had at 4000m I've felt like a f*cking god, with no problems whatsoever. Altitude is weird.