I am fairly new to going to the gym. Used to be really unfit but got to the point over a few weeks where I could run a mile at a decent pace. However went today and got tired after running about half that! Its not just that my muscles were tired, I was out of breath too and it was just really hard going. Its got me down a bit 'cos I feel like for some reason I have just gone back to my pre-gym levels of fitness. What is going on?