13-20 is still a bit of a foot race but it can be done if you are methodical in setting goals and schedules and keeping to them. There is an enormous amount to learn and do in that compartively short period of time. You may be able to accelerate the schedule by buying a lot of the necessary work (i.e. having the boat professionally set up and taking a lot of courses) but even with that, you will need to stay focused. I can put it this way, four years ago I began to think that I wanted to buy a bigger boat. Over a year ago I decided to start looking in earnest. I began negoitating on the boat that I ultimately bought back in March. The boat only gets trucked in some time in the next two weeks. I figure if I went to work on her full time I could probably have her in condition to go distance cruising by next spring. She''s in good shape and basically began life as a distance cruiser. So, If you are starting with a small catamaran which did not start as a distance cruiser you have a lot of work to do. You even have a lot of work to do just to figure out what all you need to do to the boat to set her up for the kind of thing that you are considering.
I am not trying to discourage you in any way. I am merely trying to keep things in perspective as I have seen them.