most bike systems I've seen use a 16g cylinder. Unfortunately there's lots of different size cylinders available (for lots of different purposes)- you'll need to check what your 'CO2 system' will take. The first basic breakdown is threaded vs non-threaded (although some systems will take both). If you've got something like this http://www.evanscycles.com/products/genuine-innovations/ultraflate-plus-ec015814 then anything bigger than a 16g cylinder won't fit (can I interest anyone in some 25g cylinders?), if you've got something that just screws onto the top of the cylinder then you've more flexibility as to weight.
When I finished my first half I was convinced it was impossible to go any further. But it is do-able, recuperate from the half and then start nibbling on the elephant that is the marathon training. As Millsy said, find a plan and take it one step at a time.
sounds like everyone had agreeable weekends. I was down by the seaside visiting friends from Uni- got to be a parkrun tourist to boot and cracked the 60% wava barrier for the first time in a very long time. I also caught up with the first episode of the current Dr Who.
Hope- where was the geek weekend? One of my uni friends is also a sometime figure painter.