Well I'm way behind you in the running (I'm attempting to do eight-minute miles) but ahead of you in the relativity and quantum mechanics, so I'll say this: you'll never get a proper understanding by reading popular-science type books. You want to get proper textbooks and work through the problems one by one. I can recommend Understanding Einstein's Relativity" by D'Inverno and Eisberg and Resnick for Quantum Mechanics.
Note: that last one won't get you into really interesting bits like the interpretation* of quantum mechanics, but I think you need to have a thorough understanding of the mathematics before you tackle the philosophical issues.
Note: that last one won't get you into really interesting bits like the interpretation* of quantum mechanics, but I think you need to have a thorough understanding of the mathematics before you tackle the philosophical issues.