Some PhD observations (I have one and have supervised and examined them for the last 25 years):
* The nature of a PhD experience varies hugely be discipline, by university and by supervisor, so I'd be wary of too many generalisations (including those I make below).
* Your relationship with your supervisor(s) will shape your experience massively. Are you wanting someone who handholds, who commands what you should do, or lets you get on with things? If there's an opportunity I'd encourage you to make contact with potential supervisors and talk with them directly.
* In my experience of supervising mature students doing PhDs, the commonest risks are (a) a lack of self-confidence in things academic can sometimes be paralysing and lead to kind of writer's block. Keeping an honest relationship with your supervisor and being able to express your fears when you have them can help considerably. They are there to help you. (b) other aspects of life can get in the way of PhD study, and if the qualification isn't central for your career, there is a greater temptation to let the PhD drift.
* Skills that really help in a Phd: (a) Academic curiosity - having a genuine thirst to find stuff out will get you through the low points and make the overall experience an enjoyable one; (b) organisation and drive to complete things. This often pulls in the opposite direction to (a). It's quite rare to have both these skills, but awareness that you need elements of both can help; (c) ability to synthesise. You are likely to read massive amounts of research literature, especially in the social sciences and humanities. It is impossible to hold all this knowledge in your head and make use of it unless you can bring ideas together, compare them, evaluate them, contrast them and organise them. This is ultimately what a PhD thesis will need to demonstrate.
* When applying for a PhD think about how you can demonstrate those skills, or at least show you have a willingness to develop them. Most PhD applications have a 'research proposal' section where there is an opportunity to do this.
* A PhD can feel very isolating. Try to get out to conferences, to build a network of other researchers, including other PhD students. Get your work seen and published as soon as you can - there's no reason why this can't start to happen in your first year. Feedback from peers is valuable, both in shaping your own ideas, and developing your academic self confidence.
* Enjoy it! It will be a significant milestone in your life.