Hi Dave,
I live in downtown Manhattan. The city is safe, but it is a big city. Be aware of your surroundings, and do not walk in deserted areas. I also would not walk inside the park after dark unless there is an event in the park. We walk around at night and I walk alone often, but I carry my purse securely and pay attention to who is around me. (My husband thinks I am paranoid, but I think I am cautious.)
The midtown area where you are staying has a lot of tourists, business travelers, and foot traffic most of the time. It is an exciting city. Enjoy!
Merril