Tri novice - on swimming and training in general

I have signed up for my first tri. It's Sept. 12th in the Cotswolds (England). Actually this is my first anything, only other race I have ever "competed" in was the Chase corporate challenge 5 yrs ago. It's a sprint distance: 750m swim, 20K cycle and 5K run. I can do each component individually, but I only know how to swim the breast stroke. So I am not exactly fast. So a few questions:
- for the swim: I am going to try and learn the front crawl. Had a first lesson Wed. and was thinking that I would try to build up to 750m in front crawl and use the breast stroke as a back-up if I need it. Does that make sense? The swim leg will be in a lake with wetsuits.
- I am a little confused about the best way to train. I understand the need to do every sport 2x a week and probably emphasise the swimming with 3x a week since it's my weakest category. But since I can already do the distance in cycling and running, what's the best way to train? Interval training and focussing on sprint intervals? Most training calendars have you building up to the distances but I can comfortably run 5K and cycle 20K already. Obviously the key is the transition and I have started experimenting a bit with those (30mn on the bike then a short 5 to 10mn run).
- Any other advice would be great!
Cheers! (as they say here)