Greasy Pig
Master Don Juan
I'd keep it brief and fairly formal but not dismissive and cold.
For example:
Her: "Hey, how are you?"
You: "Any fitter and I'd be dangerous."
Her: "That's great. I'm doing ok."
You: "Glad to hear it. I'm killing it at work at the moment and need to organise some stuff for a party this weekend, so gotta run. Bye."
Even that might be too chatty but it shows that your life is going just great without her. You haven't asked her any questions or indicated you want to keep the conversation going. But you also haven't come across as a complete prick.
For example:
Her: "Hey, how are you?"
You: "Any fitter and I'd be dangerous."
Her: "That's great. I'm doing ok."
You: "Glad to hear it. I'm killing it at work at the moment and need to organise some stuff for a party this weekend, so gotta run. Bye."
Even that might be too chatty but it shows that your life is going just great without her. You haven't asked her any questions or indicated you want to keep the conversation going. But you also haven't come across as a complete prick.