I don't understand why I can do 4 miles at a 10 min/mile ace, yet can't run 8 miles at a 12 m/m pace.
Seriously, you need to not obsess over this. First, 8 miles is a lot longer than 4 miles. Doubling your mileage is always more than twice as hard. Second, it's apparently still very hot where you live, which makes it even tougher. And finally, like you said, you've still got some extra pounds, which makes distance running
way harder. You should be thinking about the weight you've lost and the fitness you've gained. You know this already, but most people with a 31 BMI would be completely unable to run a single mile, let alone 4, let alone 8. You are in the process of making a major lifestyle change. It's already shown dividends, and it's only going to get better. Just stick with and don't get down on yourself. You're absolutely doing the right thing and you should be feeling great about the results you've achieved so far, not bad stuff that you'll be able to overcome down the road.Edit: For real,