Q: Is it OK to split my long run into two segments that I do during the same day?
A: The answer, in short, is no. The whole point of your weekly long run is to teach your body to adapt to running for a long time so that it can do so efficiently. Running long over one continuous time boosts aerobic fitness.
The purpose of this is so that you can develop the endurance needed not only to race well, but also to get the full benefits of race-specific workouts that you do during the rest of the week.
Another thing: your Sunday long run is practice for race day in the other areas. That means that by running a distance similar to what you’ll be racing, you find out how much fueling you need, what type of rest and recovery work best and you can develop tactics to conquer mental and physical challenges.
Bottom line: if you’re going to ask your body to run long, you have to first condition it to do so. However, if the long run is becoming a dreaded workout, there are ways to switch it up!