Zone 2-3- do you eventually run faster?

I have run 4 half marathons over 4 years. I’m overweight, but really enjoy running. I just go out and run. I recently learned of running in zone 2-3, and I did it this past weekend. Kept my heart rate under 160 for 2 hours and I felt freaking incredible. My Pace was 14:55, but like I said I felt great and could have kept going! Eventually do you get faster? How do I train ? Please explain to a 5 year old…I can run an 11:30 pace for 4 miles then I’m done. Not hurting, but done!
I’m running a marathon in December and would love to run it under 6 hours.