If you work 9 - 5 I am assuming you must leave for work at around 8ish and get back around 6ish? So thats 10 hrs, that is far too long to leave a puppy or any dog.
The days when you work 9-12 the pup would be fine, can you not reduce your hours maybe to be at home with the pup? I know it would mean sacrificing something else but if thats what you want you have to plan your life around them.
I work silly hours now, I do long shifts from 4pm - 2pm so that Im home all day with my dogs, then leave as my partner is coming in and they have him for company on the evening.
My dogs are of course left sometimes but anything past 5hrs I start to feel uncomfortable, and I would nt leave a pup more than a few hours anyway.
Alternatively have you any doggy day care centres near you ? The my petstop chain have one near me, I was there for something else the other day and was watching one of the day camps! It was great, they had 2 spaniels, 3 terriers, 2 crossbreeds and a most beautiful doberman all playing football together inside, they were loving it.
Unless you can drop your hours or get someone to look after your pup on the days when you work I would say leave it until you can be at home more, its not fair to the pup or yourself, as youll just worry about him/her and it will spoil your enjoyment .
Puppies are very time consuming and at first need constant attention, like someone else has already said, I save my annual leave up at work and usually have a full month off with the pup to start with to get him in a routine.
Hope you can work something out, if you have family who will have him a couple of days youll be fine!