I admire your courage in pursuing this, and your insight into your own likes, goals, and potential enjoyments and rewards. It's definitely not too late. When I was 35 I walked out of a tenured professorship in Philosophy at a private university into ... well ... over time, something very different. I don't even remotely regret it. My wife did something similar. My daughter (age 36) just left her teaching career for work in business and is very happy. So go for it. From what you say about yourself, I'm sure that you can be an excellent repair technician.
You really do need some training -- and actually, a kind of apprenticeship. This is tough because most people run small (usually one-person) shops and can't afford to pay an apprentice. One thing you might consider is biting the bullet, keeping your current job, and putting all the effort you can in to "after hours" work/training/education. You might even offer to work part-time for a local technician for no payment if he/she will train you as part of the work. (I did something like that in getting ready to leave my university job, but since I was going in the direction of computer science, pay wasn't a problem!) Think of it as paying for training with your work.
You need training in both the technical aspects of instrument repair specifically, and also in broader areas such as tool use, machining, soldering/brazing, etc. You used to be able to get that in community college, but it's become almost impossible (except for welding) because they've all dumped their manual trades departments in favor of "computer classes" and computer controlled technology.
You CAN do it, but it won't be quick, and there may be several years of transition where you need to continue to support yourself in your more traditional job while gaining those skills (which include business skills, since you probably will end up running your own repair business and maybe contracting services to music stores, schools, etc.) Another consideration is that if you want to set up your own shop you'll need thousands (probably tens of thousands) of dollars worth of tools, including very specialized tools. Moving in this direction takes a lot of faith and courage. But if you want to go there, you should go for it. One step at a time.
I'd also suggest you post your goals and hopes on Tubenet and ask for advice there. There are a number of very good independent repair people who read that on a daily basis. Or shoot an email to Dan Schultz (http://www.thevillagetinker.com/), and ask him for his advice. He moved from being an engineer to being a repair tech some years ago, and he's a really nice guy.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)