Restaurant near 6th Ave and 25th St
Penelope on 30th and Lex is probably just within your distance parameters. good comfort food, not too expensive. people think of it as a brunch place but i like it at dinner on week nights. much much less crowded.
Boqueria on 19th b/t 5th-6th ave for good tapas though you can't make reservations.
Bar Breton on 5th b/t 28-29th -- i haven't been but it always looks pleasantly full when I walk by and New York Mag mentioned they had great burgers in the "where to eat in 2010 issue"
I highly recommend Bar Breton. The chef/owner is Cyril Renaud, whose previous restaurant was the superb Fleur de Sel (and sorely missed). At Bar Breton, Chef Renaud's cuisine is not as haute. Nevertheless, it is just as delicious. Service is excellent, and the ambiance is extremely pleasant.
Photos of one of our meals at Bar Breton can be seen here: http://www.flickr.com/photos/11863391@N03/sets/72157616100137962/