The difference is subtle. Won't - definite and certain. Wouldn't - hypothetical. Both are correct, but have slightly different meanings: "If the meeting runs late, I won't be able to make it." (I definitely won't be able to make it if the meeting goes late.) "If the meeting runs late, I wouldn't be able to make it." (Hypothetically the meeting could go late, and in that case I wouldn't be able to make it.)