Not hard at all if you take the time to study the subject.
If you really have a good grasp of the concepts sometimes the hard part is the drudgery of possibly having to write all the device drivers for the various devices and peripherals that the RTOS has to service.
In the case of the OP, he seems to be talking about rewriting everything from the most basic device drivers on up to bootloaders and even the GUI. That's a ton of work and it requires knowledge across a bunch of areas he is not yet well-versed in.
Also, when it comes to the idea of writing an RTOS, there's a huge difference between an RTOS for, say, a non-mission-critical device of some sort and something that could kill somebody (plane, powerful machinery, etc.). That is hard not because the concepts require superior intellect but rather because you really have to understand the code and potential issues behind how you wrote it very well and test like a maniac.
I have written RTOS's for embedded control of devices that could take someone's arm off in a fraction of a second. Hard? Extremely, when you consider what the stakes are and particularly so if it is your own business, your own money and your own reputation on the line. There's a lot more to programming that bits and bytes.
