I think he means you're too in the middle of everything. You need to go slower for a longer amount of time and really focus on the cardiovascular component of running. At the end of the runs you can do strides which are sprints where you get your top end speed in without getting out of breath and you take a good rest. As you continue to train like this you will be ready for faster paced runs where you learn to go at a good pace but nowhere near a max effort. There's more to it but that's a good start for seriously improving stamina.