You go Brother! I know to many of my friends that hit 40 years old and just turned to mush, never do anything active and have no energy. Now I am not the fittest person in the world but I keep myself busy in the yard almost ever weekend, I shouldnt say yard, more like forest! I walk my dog 2 miles every morning before I go to work and drive my wife nuts cause when I do have down time, I never can sit still!
Its not a mis-life crisis, more like a mid-life awakeing! Go out and just have fun, sometimes that means bruises and sore muscles but thats what its all about!