HMG,
don't worry about your swimming as lots of people do breast stroke, just be realistic about the time you put down to complete the swim in!
Enjoy the bike section and again don't worry, you out to have fun!
The run, you'll be surprised how many people you pass!
You should be fine to do a triathlon in May after an April Marathon. I did one last year 2 weeks after a marathon.
Go for it and have fun!