Hi Izzy,
Firstly, an idea of what you are on now will give a better insight into how much it's going to hurt financially. Will you be able to pay for training outright or will you need to borrow? Does your family situation allow you to move to a remote area for a prolonged period?
As to regional vs major, there is no need to set a goal for one or the other. Aim to get as qualified and experienced as quickly as you can and then take what comes. No-one is guaranteed a gig with the majors, and whether there is scope for that has a lot more to do with Wall St than with your age.
Also bear in mind that you are obviously doing this for personal satisfaction. Will you be satisfied being the oldest guy on the crew and still a second Officer? Would even the prospect of being a 55YO
VB F/O really appeal, or would a slightly lower wage (perhaps living in a part of the country with lower costs) but getting to be a regional captain be a better aspiration?
As to whether to go for it or not, I lasted exactly 4 months in an office job, and can't imagine how someone who obviously has the imagination to want to fly could have lasted 23 years!! GET OUT OF THERE AND GO FLY!!!
If you wind up a career instructor, you won't starve and will still be miles ahead in terms of personal growth and satisfaction.