There's been a lot of good advice here, including that about training slowly over the longer term for us older hikers. Practicing pitching your tent/hammock and practicing sleeping are also worthwhile. And while it's not really physical training, dialing in your rain gear is important, too. Once you learn to comfortably handle rain, it makes dealing with crappy weather a lot easier. Nobody ever complained about having a dry sleeping bag at the end of the day.