How to increase speed of long runs?

Hello friends! I posted here a couple days ago about starting my marathon journey. I am a very very very slow runner, im talking 13 min miles on my long runs, without walking. I currently tap out at about 2 miles when I have to walk, but I think that's more of a mental thing that I have to work to get over rather than me being tired. My garmin tells me I start pushing into Zone three at around 11:30 minute miles but I feel like that is so slow.. I know I shouldn't compare myself to people on social media or strava, but I see all my friends doing 8:30 minute miles for 4 miles easy peasy and it's a little discouraging. I've heard you either work on speed or base but not both at the same time. Obviously I'd rather work on base for my marathon but I was wondering how do people lower their heart rate while running so they can lower their paces?

I know I have ALOT of work to do before decemeber, but I was wondering if anyone had any advice.