Long distance mountain bike multi-day bikepacking events are my main focus so …
Don’t worry about FTP, I’ve been (well) below 3W/kg for most of those I’ve done, currently a tiny bit lower in both FTP and weight than yourself. What you really need is time in the saddle and the ability to just keep going.
Treat indoor training as an activity in itself, pick a plan or sequence of plans you like the look of and that lets you work on something you’d like to improve. There’ll be some crossover/benefit from pretty well any of them. Maybe even a Low Volume plan and do long weekend rides (assuming that’s legal in your current jurisdiction) with a bivy.
Is 10km/h your average speed or just up hills? Hopefully the latter. Assume 20km/h that means 13hrs on the move per day for your event which isn’t much, I reckon on 16-18hrs to give plenty of rest (comparatively, reduced sleep is expected on events like this).