I have the ever-so-useful London Underground toilet map printed out, and a copy of the general tube map covered in little numbers and arrows.
I'm starting at Catford Bridge at 07:14, meaning I'll hit the underground at Southwark at 07:36. Then I'm heading roughly clockwise, to Waterloo, changing for the Northern Line to Embankment, then District or Circle to Westminster, then Circle or District to Victoria, on to South Kensington to change to the Piccadilly and up to Kensington Olympia for my first terminus.
Depending on how busy the tube is by then, I'll either head down to Heathrow or up to Ealing Broadway. I think Heathrow will be fun - I've never been out there by tube before.
Anywho, I'm not going to give my whole route away. Needless to say I'll be topping up with tea along the way, pacing myself with sandwich eating, and hopefully hitting the stations with toilets as required.
Off to bed now.