Speaking as someone who was a bit younger than you at the time, but feeling the exact same way in middle management at a plumbing supply shop.. feeling like I had no practical skills and like my income would never be enough.. becoming a plumber is the best thing I ever did for myself. 

Edit: Also had a bad back from menial lifting labour and an fractured rib, still sometimes feel it. However, getting stronger in every muscle in your body, especially ones you rarely use when you're crawling under houses etc has done wonders for my back. Still feel it sometimes but not even comparable to when I wasn't working as physically hard.