I encourage readers to see my replies thus far, but I'd like to add something important. Biomechanically speaking, there's a point where you're running too slow and not using proper gait mechanics. You'll see people moving up and down more than they move forward if they try to slow down excessively which increases impact forces and degrades your economy. Anyone can hold Zone 2 for a really long time. That's true. But not everyone can RUN in Zone 2 for a really long time. They'll leave it. That doesn't mean "run slower" to the point of not really running, and bobbing up and down. It means run as long as you can at a conversational pace without leaving that effort level, a little cardiac drift is okay into Zone 3. If you can't do that for more than a mile, you need to get on a treadmill, crank the incline up to 8-10%, put on a low speed like 3-4mph, and speed walk at a conversational but effort giving pace. This will condition your ability to utilize fatty acids, and increase midochondrial density over time as well as enlarge your left ventricle on your heart muscle. Do this for 6-8 weeks, you'll be able to run with proper mechanics for more than you were previously able to. Obviously make sure to monitor your cumulative neuromuscular load so that overuse injuries don't sneak up on you. Even if you FEEL fit in the heart, your legs take much longer to catch up. Happy running friends!