I do triathlons too, and I could see where your concern might come from, considering it is time consuming training. I am starting medical school this august and I am really nervous that I won't be able to put full effort into training next year, but I've spoken to a good amount of medical students and residents that find the time to get it done. You can do it, it's just a matter of making it a priority over the other things you might want to do in your free time. I think triathlons are a great way to stay in shape, and if you do them competatively like I do, they are a great way to burn off some steam! This will probably even help burn off some stress from med school as well.