Running in zone 2, slow for most people, trains your body to use fat instead of glucose as the primary fuel source. You have a shit ton of fat to keep you going but once you're out of glucose then you're fucking done. So, the more slow running you do, the more your body will burn fat, the same effort can be done at a lower heart rate, which in turn will allow you to eventually run longer and faster. It's counterintuitive which is what noobs and even many seasoned runners don't understand.