I have been in a similar situation. I have the Speedplay Frogs and was feeling uncomfortable clipping out. No problems clipping in. I fell once, was clipped out but leaned the wrong way, and then the rest of the time practicing I didn't fall, but I was so tense, just was miserable.

I decided to wait and try again later this fall. I just completed my first metric century (actually it ended up being about 66 miles, 15 mph ave pace in a hilly, for me, course.) I am doing my first full century ride in Sep. I figured I might as well get through those rides with my toe clips for now and try again after that in the late fall. I usually have a 15-16.5 mph ave pace depending on the ride and who I am riding with. I know the clipless will be great and give me more power, I just have to mentally be ready for it.

One thing I have been doing to get ready for it, is practicing pulling my foot out of the toe clip with a little bit of a turn (as if I would need to do with the clipless) a few minutes before a stop. I was so used to pulling my foot straight back but now I am just trying to turn it at the same time to create that memory when stopping. I also found myself letting my foot hang and reaching for the ground often. Instead of doing this, I now step forward and off the bike at every stop, it makes me feel so much more stable vs trying to reach the ground. (which I can barely do because of the height of my seat.)

I like your idea of getting used to it slowly and not going on a super long ride with it in the beginning. I am going to do the same once I eventually get used to them.

Good luck with the clipless! I can't wait to start using mine without fear- I will try it again in October.