Anyone eating close to 5k in the offseason is probably going to be starting his diet at close to 500g, especially easy if cardio is increased concurrently.
I really like finding a "base" amount of carbs per meal, then doing 150% of this amount of first meal and post workout meal, and doing 50% of that amount for last 1 or 2 meals, depending on training time.
This is one of Chris Aceto's methods.