As a recent PhD entering jobhunt, should I agree to host a weekly seminar?
It's in your interest to focus on research, then job search, then teaching, then service. This is service, so lowest priority. See if you can get a student assistant (grad or undergrad) to do a lot of the grunt work, while you handle the higher level logistical stuff. If you can't get some help, doesn't sound like it's worth your time.