Hi,
I took that train myself in Sept. 07, and I recall it being pretty full. Please bear in mind that you may get assigned seat numbers, rather than choosing your own. The queue starts forming about 45 minutes to an hour beforehand. The queue starts in the main hall, then you all stagger/walk/march/run down a long tunnel to your track. The tunnel is open anyway, and the queue is a kind of device to ensure passengers are in the correct place. If you could find out in advance which track the train was departing from, you might be able to get a head start.. Red Caps often give the elderly and infirm priority boarding, if you have youngsters and baggage you might qualify also. Boarding takes place about 20 - 30 mins before departure, if the coaches are ready.
Roller blades, cycle, small motor bikes might be worth a shot too!
Ed B)