getting a PhD is not about being clever, and those who have a PhD are not automatically cleverer than those who don't.
fundamentally, qualifications standards are:
GCSE - has basic literacy in the subject, can learn and retain simple facts and apply simple techniques in the field.
A-level: somewhat more in-depth understanding and able to understand and use more complex techniques.
Degree: able to follow and reproduce information able the development of the field and could read about and understand current new discoveries, able to apply much more complex techniques and retain a lot of information about the topic.
Masters: more in-depth thinking about "why we know what we know" in a field, including much more engagement with reading up on current research, because this is the best building block towards being able to confidently teach it (up till a few years ago, anyone with a Masters degree could skip doing a PGCE and was considered a qualified teacher on the basis of having a Masters) or to develop the skills for the next stage.
PhD is the level at which you show that you can apply this knowledge independently to something new, where there isn't yet a "right answer" that can be graded, but can do the work to add a brick to the edifice of human knowledge and understanding in the field and can defend your reasoning as to why that snippet of knowledge could be considered correct. holding a PhD means that you have proved you have the skills to do that and need no further direct supervision, you can now work in research and generate new knowledge which then goes through peer review for quality control (and you should be able to be trusted to peer review the work of others too)
in terms of skill and intelligence, a lot of people who leave university with a degree and go into a profession where they use and apply that knowledge and also have to tackle new complex problems where the answers aren't already known, are already functioning at PhD level. it is not to be feared. a PhD should simply be seen as completion of the apprenticeship phase for someone developing a career in academia. you don't need to be a genius, just capable of working hard, reading and understanding the work of those who have been working in the field before you, and developing that thinking by one more original step.
you sound perfectly capable of doing this. go for it.