I'm like Kay, at first I was hard pushed to get 2 litres down me but now I can manage at least 4 litres - as long as I drink it throughout the day and don't try and force it down in the evening when I realise I haven't drunk enough.
I keep 3 500ml bottles in the fridge and drink them as and when I want refilling each one when I finish it.
Good luck.
x