Congratulations on finding an institution which will take you on straight away.
I suspect you would find it difficult to work and study full-time; I assume you weren't thinking of doing that, but were thinking instead of doing your PhD part-time? That would be quite possible; a History-teaching colleague of mine at school did exactly that. It did take her about 8 years though. That doesn't mean it would take you that long, but it might.
It's not just doing the research, you see. You will have to develop your skills in conducting research; generating and analysing data, getting to grips with relevant, and sometimes irrelevant theoretical frameworks and epistemologies. All this will take time. I don't know what programme of support in this area your institution has planned for you; if it has nothing, and you haven't had a go by doing a Master's, then you'll be finding your way more slowly.
To be honest, I think this is going to be a case of 'suck it and see'; no-one can tell you better than you know already what your personal capacity is. It's going to be tough, but it's a very rewarding thing to do.
By the way; you say that your PhD has nothing to do with education, but I'd be very surprised if you didn't become a better teacher by the end of it all. However good you are now at analysing sources, reflecting on meaning, synthesising information, contextualising knowledge within a field, etc, you'll be even better at it after your PhD.