Depends who you are and where you've come from - I'm in industry and doing mine part time, in 3 1/2 years I've published 5 papers (4 conference 1 journal) and have 2 more in the immediate pipeline. I think I can finish in the minimum for a part timer, and if I was full time I think I could have wrapped it up in 2 years - maybe. I'm no genius, but have been in industry 20 years, so have a firm background in my field. I'm pretty much used to doing a block of work and writing a report in my normal life, so experiment and publish really works for me. Plus I'm old so very mindful of time...
The worst bit for me is the time you waste at the begining, looking for direction...

My situation is different, but I think I can answer the question.
Do you really want the career path the PhD will open?
Can you cope with the fact it will overrrun?
I was 38 when I started my PhD, Part time, job, wife, 3 naughty kids. I'm 18 months in and get the highs and lows every PhD student does.
I don't need a PhD for my career, but thought there was one in me, so thought I should do it.
"I'll be almost 29 if I finish on time" - you need to get out of this mindset - your life can and should go on during the PhD. I've recently made a transition from the "prison sentence" to "this is something I do now". Probably when I realised that 4 years (Ha Ha) would probably be 7.
Financially I'm not in your situation, as I have the house/mortgage/car/kids/dog, but certainly the opportunities in my Uni for Postgrads to lecture, and undertake RA work are numerous (although I don't know your discipline).

My top tips
Get to know your supervisor(s) - if they go to the pub on a Friday night and you get the invite, go. This is where you really find out how well your doing.
A difficult supervisor may just be trying to get you to defend your work. Get ready for criticism. It will be harsh.
Your supervisor will have seen many PhDs through. They know what they are doing and allow them to steer you a little.
Mental strength is more important than mental ability

"A big reason I'm doing my PhD is to get the lectureship position - if it's there at the end of the day of course. Having a PhD will help getting a lecturers position but do I really need to be focused more on the research side of things? I realise having such an outlook will probably mean a lower paid position at a post-1992, former polytechnic uni but if it's what I enjoy from the job does it matter?"

Why do you say that?

I failed my A levels at 18, did an HND (with no entry quals), then did a part time BEng at a "former polytechnic" and got a first.
I'm now at a Russell group University, and I could get a job there, (express an interest and a good supervisor will help you), but would equally work at a former Poly.

Don't go into it on a downer! Want it and get it!
Andy

Only just started, but I love it! I have a family and a job, but when I walk into the University I feel like I'm coming home (despite the fact I am 38 and my hair is not green)

I'm in a similar situation - 3 kids, and I'm doing it part time. (And I'm 38!) I did my degree part time 10 years ago and was obsessive, so my family are ready for the pain. I'm (fairly) undaunted by the prospect of all the work, really because I was under no illusion it would be easy. Part time and family, something WILL give. It's better to know beforehand what that is..