Excellent question. While I'm not a marathoner I am a half-marathon runner and have some experience with this.
The Pebble/Pebble Time supports showing running stats on the watch when synced up with RunKeeper or Endomondo. There might be other apps available but those are the two I've used.
They require that you have a smartphone drive them, and you'd have to have enough smartphone battery to not have the phone die while using GPS for your run.
Both apps let you start/pause/stop your run from the watch. I used to use this for biking (and a bit running) and it worked out really well.
Note neither currently has a built in HRM, an so you'd need to wear Bluetooth HRM to sync with your phone if you care about that stat, but they CAN show you your heart rate on the watch.
I've since transitioned to just using a TomTom GPS Runner or Multi-sport with the built in HRM as that works well for me while running. It also removes some of the battery anxiety I used to get when using my phone for GPS.