I liked many of these responses. My advice is to gradually build up to longer trips. I walk up the hills in the neighborhood when it is raining and i can't go out hiking. With a pack and boots if I'm getting ready for a long trip. I hike once a week in the woods, some times with considerable elevation gain, mostly easier trails. Sometimes over night, mostly day trips that last 6 to 8 hours. I am older than you and because I train regularly younger people never fail to mention how inspiring it is to see an old guy doing so well.
Jim M