Excellent advice Qing.
My general advice would be to work your way up. It would not make sense to go from doing a few 1-2 mile swims and then try 20 Bridges.
If you have done a number of 5k swims with a few 10k’s then you are better prepared. What helped me a lot was to do a couple of swims that were at least half the time of your expected long swim, ie in the order of 4-5 hours. Some of the 8 Bridges swims fit that description.
Good luck and happy training. It is a remarkable experience.