Congrats on your new triathlon journey at a young 50:-). Besides what Coach Teresa and Deb recommend you might consider signing up for a local triathlon open water clinic. OW clinics would cover and simulate both time trial and mass start swims, sighting, relaxation methods, swimming in groups and so forth. Of course designing (race goal) practice sessions in the pool and learning good technique are important and applying them in the open water will be helpful before race time. Perhaps joining some locale open water swimming groups or rallying up others who are looking to open water swim. Books, Videos, online courses are all helpful tools. But they won't replace the valuable hands on experience and real time feedback of having a good TI Coach.