Getting back in the swing of things

Finishing a triathlon was a great feeling, followed, of course by a major let-down and a decrease in my training (why???) But I've just set a new goal - complete a 1/2 marathon in May. My training will soon begin and I can get back on the road to seeking fitness and sanity. I find that the more I exercise, the saner I feel. Endorphins are probably the best anti-depressant around. God's choice of chemical?? Scared about the 1/2 only because of my age and injury potential. But, if I do everything "by the book", it should work out, right? And help me shed the weight, which ironically, I didn't manage to do during the tri!