If the Catalina is in good shape, and it works for you, then go ahead. I understand the idea of starting in a dingy and working up, but a C25 is not really that big to start with if you get a little help and do a lot of reading.
The question not asked, and that rarely is, is what is your long term plan...if any. It is great to start in a dingy, move to a 2o something, then a 30 something, and finally a really big boat so you can start downsizing when you are older. The problem arrives when you are already 60 something and by the time you arrive at your destination you cannot handle the dingy let alone the big boat. C25 is a great little boat and you can learn on it. Have at it.