Bogey, I'm 41, and I'm still waiting to work out what I want to do when I grow up. And you know, there are loads and loads of us out there, but people tend not to admit it. Not everyone has clear focus and knows what they want to do from before they choose their GCSEs.
It is harder when you're older and have more commitments in life, though, as you've already found with things like tour leading.
Think about the things that attract you to tour leading - is it interacting with lots of people, passing on information and educating people, the history, or...
Stop thinking about job titles and roles for the time being.
What behaviours do you find most satisfying?
What do you really hate doing? Decision making, as a start. 
How do you feel about things like helping people, planning, organising, learning new skills, handling money, IT skills, persuading others and so on?
Do you like using maths, writing, practical work or theoretical/conceptual?
How do you feel about manual tasks, like lifting, moving around a lot? Office work, outdoor work?
What practical things do you need out of a job? Presumably you need some stability, because of the family. Full time, part time, office hours, shift work, on-call*? Work from home, single office location, travel in local area, country-wide, international? Team work, working alone, supervising others?
Think about all these things and more, think about all the things you've done in your life - work, family, study, voluntary work, personal projects (e.g. planning a big party, decorating the house) - what things did you find most satisfying, and what things most wound you up?
You should end up with a list of things you really want, and some you really don't, and from this, you should try to build a profile of your ideal job, and then look at which jobs will actually fit that profile, and then you can look at what you need to do to retrain.
- I've just had a really bad weekend on-call. I don't recommend jobs with on-call just now...