trouble increasing running distance

I've used calorie count to lose 20 pounds over the past couple years (slow but steady!) and am generally happy with my weight. Now I'm just trying to find my way to better fitness. I've really started to enjoy running but am having trouble increasing my distance. I seem to max out at 3 miles and just can't go further. I've experimented with eating different things at different times before running assuming it's an energy thing....but no luck. I tried slowing down to see if that would help but it really didn't make much of a difference, still pooping out at around 3 miles. Any tips on how to break through? Thanks!

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