I had an acdf of c5-c7 with allograft, titanium plate and screws. I am 39 years old. When can I run again? I do half marathons.

Fused? Once you have solidly fused ( 3-4 months ) you should be able to resume running. Talk to your surgeon.
Post-op instructions. When you were discharged from the hospital you should have gotten instructions from your surgeon explaining what type of activity you can participate in and when. I bet the answer is in there. If you have questions, call the next business day and ask to talk to your surgeon or his/her nurse for clarification on activity level.
Ask doctor. Your surgeon is in the best position to advise you when you can resume running. Everyone is different and restrictions vary amongst different doctors and clinical situations. Sorry i cannot be more helpful.