Trips like the GC to Bris show you can do these sorts of trips in a SOT. True there is not the same storage space as a SIK but that just means gear management. Meals are pretty easy to sort and only go for things that are dry and you add water to, even bread is too bulky for longer trips. Dehydrated is good but you can do with tuna sachets and noodles if needed. I actually make up day bags which include my meals and snacks for each day. Typically trail mix makes up lunch. Food for these things is an art in itself. Water needs to be calculated and rationed in several smaller bladders so you don't lose it all with a single puncture. Lots of small dry bags to store your gear. A small midgie proof tent. And study the area you intend to go to.
The big difference with going solo as opposed to a group is to make sure you can self rescue or be visible. So investing in a VHF, flares, signal mirror, fire flint, PLB, etc would be a good plan. If you look at one of the Jumpinpin trip proposals you'll see a checklist attached. Everyone's list varies but it's a good starter.
Just saw you are talking freshwater, so I assume a river. 120km in a river is a long way.
Yak PB's. LT 110cm, Spanish 105cm, Spotty 93cm, Snapper 65cm, Grassy 53cm, Flathead 70cm, Tailor 41cm, Jew 70cm,