Never redeemed any miles so I can't say what the norm is for that route which I fly a lot and is always packed.

Try Denver to Hartford which is less busy. Its a 90 minute drive to NYC. Alternatively, if focused on public transportation, you can catch a shuttle to the train station and then Amtrak to NYC. Not the most efficient route but an option.

I believe F9 also runs DEN - PHL but haven't flown that route.