I just went for the first thing that popped in to my mind that is a primary reward -- something biology has conditioned me to want, and will activate reward pathways (quickly!) when ingested/experienced.
Honestly, you could probably just start with the secondary reward of congratulating yourself -- it's just important that you do it out loud and call yourself by name (eg, "John, you did a good job of running today"), so you hear it happening, instead of it being part of your mental dialog.
Generally though, I find that speaking to myself is psychologically different than talking to myself internally, so YMMV if that's not true for you.
Honestly, you could probably just start with the secondary reward of congratulating yourself -- it's just important that you do it out loud and call yourself by name (eg, "John, you did a good job of running today"), so you hear it happening, instead of it being part of your mental dialog.
Generally though, I find that speaking to myself is psychologically different than talking to myself internally, so YMMV if that's not true for you.