Eventually, running will be a reward in itself. But you're probably far away from that nauseatingly perky attitude right now. Instead, think of something you really really desire. Once you've completed 2,5 km of non-stop running, you can have that thing. I wouldn't make it an edible or drinkable reward but that's me. You decide what you crave the most. If you want to, post it here, as I'm chronically curious.