I was a swimmer first then moved on to triathlons. Like _ForceD_ above, I've done a lot of long distance open-water swims. For training, I have always found swimming alone was difficult (I had 1 year where almost all my swims were in the pool by myself, as there wasn't a group to swim with). The easiest way, in my opinion, is to swim with a group. The best is with a coached workout. With running and biking, just training more (and smart) helps a lot. With swimming, form is so critical. Swimming at a coached workout provides both some people to swim with, but also feedback on technique.