In reply to Rampikino:
I think, compared to you, I am lucky. The second movement occurs whilst I am still at home, fortunately for me, but otherwise I follow a similar pattern.
I attribute this to the adrenaline/nerves and not something I can (or necessarily want to) do anything about. It does not happen if I am just lazing about the house that morning, so it is caused by the fact I am going running.
I never need to go again after this second movement at home, so I am usually fine for the event (even if it starts a couple of hours later, following a train journey, warming up and queuing for a pee).
I don't know what you can do about it - get more excited/pumped up whilst you are still at home? Get up even earlier and go for a quick warm-up run? Have "a couple of tabs and a coffee" as Hardonicus suggested?!