The Cairncorms
The cairngorms bike packing route what can I say about this one, what a wild adventure, I am adding this to the challenging routes, but it could be intermediate if you remove the inner circle as that is mostly just tough hike a bike. If you decide to remove the inner circle, then a gravel bike should be sufficient. However, following this route in the official way it gets very rugged, the terrain is more suited for a full suspension enduro bike. Very good bike handling skills are a must if you want to actually ride a big chunk of this route.
The rest of the route is quite nice, but it is a wild adventure in Scotland, so bear in mind the weather can change fast and trail conditions can be unpredictable. There are a couple places for restocking, but you should be able to carry a day or two worth of food to be on the safe side, I ended up staying in a bothy one night to hide from a storm, There are a couple bothies on this route one is quite popular so might be busy in the summer months. So bring you tend to enjoy some great wild camping spots along this route.
Just under 200 miles for the full route, but it can be very slow-going, so around 3β4 days if you have a good amount of experience.