So with 1d6 HD and 19 con, you have (4+4)+(3.5+4) = 15.5 HP/day.
Add +1 con and you gain +2 HP/day/HD (each +1 on average gives +1 HP/day/HD, but parity gives you an advantage).
Add Durable (+1 con, min roll 10) and you have (4+5)+(10+5)=24 HP, or 8.5 HP/day/HD. The non-+1 con part gives 6.5...