Departing AA it doesn't make much difference which terminal your flight leaves from. From the Hyatt Regency you'll walk to Terminal C, period.
Check your bag and clear security in C. Once past security, the airport train will take you to whichever terminal you need.

Where the Regency is NOT feasibly "walkable" is when you need to BOTH check luggage and are not flying AA.
Once at Terminal C (if not departing on AA) you would need a shuttle to your airline's terminal ... you may as well take the shuttle from the hotel.