I'm getting into a better routine of training regularly and I know I'm getting more fit. I can go for longer at the same rate of exertion that used to blow me out after a couple of minutes. I haven't tested myself recently, but I know that my HR at AT (implied by HR at VT) is higher than it was when I started training a year ago.
(AT is the Anaerobic Threshold - more modernly termed the Lactate Threshold - the point at which lactate starts to build up in the blood; VT is the Ventilatory Threshold - the point at which you start breathing much faster than you were at only a slightly lower level of exertion. VT is much more easily measured than AT or LT, and it serves as a proxy measure for them.)
Here's my question - what is different in my body from a year ago? Do I have more mitochondria? Are they more efficient? Is the chemical balance in my body different? I know the effects of training - I want to know what is actually changing as I get more fit.