Same decision... starting in the pool
Though I am currently trapped in NYC urban exile and jealous of the trails near you. I, at 49, have made the same decision.
I have little choice but to start in a pool and don't regret it. I also work really hard not to kid myself about the huge difference between the large bath tub and a dynamic body of water. I don't take breaks, I don't stand up, and I don't push off of walls all trying to build up enough confidence to feel like I can be safe in open water. (which I now do, and this spring I will test it).
I've learned swimming is much more skill intensive than either the running or road riding I do. (In the past I did a lot of mountain biking which is also very skill-intensive). So jump in the pools and really focus on your technique. Does it really matter if you're out in the woods or on a terrain park? Same skills you need to build, and keep real about your circumstances.