Do this at least a week before your trip in case you need to amend your plans!
Lay out everything you intend to take on your trip and check it off your list. Then try and pack it into your selected bags. When packing, keep items that you are likely to need on the road accessible, in particular bike tools, spare tubes, pumps, sun cream etc.. If after several attempts you cannot get it all in, you will either have to change bags or get rid of some stuff.
Once the bag(s) are packed to your liking, get someone to help you haul them to the bike and load it up. Hop aboard and ride a couple of miles down the road and back, preferably taking in at least one stiff hill. When you get back and have recovered sufficiently you can decide what you can do without!
Seriously, if you are not used to touring make sure you test ride the load before the start. Be careful because the bike will feel very different with a lot of extra weight, especially when setting off and around corners. Your braking will not be as sharp either!
Equally, at the end of your Lands End to John O’Groats ride be prepared for how weird the bike feels when you take the extra weight off, after your body has adjusted to it over several days. Bizarrely it is almost the same change in handling as when you put the weight on. It is much easier to ride though.