In this case you're probably making the best choice. My favorite Cat had a tumor blocking the digestive track and their wasn't enough time as she couldn't get nourishment. The Cat was 20 years old so we figured it would be the best choice.
The Cats cover up their illnesses so they could be almost stone dead and you wouldn't know it. By that time it's time to rush them to the vet. The end result is the same and it could cost a ton of money to keep them alive for a little longer. The Cat before "Squiggles" cost $5000 and died within a month anyway.