Where's the show? I looked and found Google Map entries for two "Milos" restaurants on 55th or 56th - both bet. 7th & 6th Ave. Depending on the theatre, how fast you walk, your route, and skill in navigating crowds, getting to the Broadway theatres can be 5 minutes for a fast walker (Studio 54) to 25 minutes (Nederlander, through the most crowded Broadway sidewalks).
What are the courses? A common 3 course, pre-show set menu might have a pre-plated starter, a main course and dessert. If you know what you want and tell your server you've got a show, they're usually pros and without hurrying you can get out in 25 minutes.
So, it depends. In any event, you DO want a relaxing evening so if it looks tight and you can change your res, it may be wise.
You may want to call or email the restaurant.
...and in any event, I'd recommend avoiding the Times Square mob and backtracking a bit to 6th Ave (Ave of the Americas) and walk down 6th until you reach the theatre's street for anything below 48th - wide sidewalks with little pedestrian crowding. Especially for the easternmost houses: Cort, Lyceum, Belasco & Sondheim theatres. (Avoid 7th Ave between 47th-45th.)
Re-reading your original post, I notice you're thinking of stopping by a second place for a pre-show drink. That mob scene can be as bad as the sidewalks for the unwary. If you tell us the show/theatre, we may be able to help you with suggestions for the bar as well.
Updated On: 5/22/16 at 03:29 AM