That endorphin hit is truly addictive. Last weekend I went on a pretty strenuous ride in the morning, and that afternoon, legs still wasted, I saw some people riding while I was a car passenger and had a damn strong urge to go ride some more. On the other hand, I've really upped (doubled!) my riding lately and took Sunday as a rest day. My body's glad I took the day off.
I'm formerly sedentary also. Sedentary people tend to associate exercise with physical discomfort, pain, and often embarassment. Exercise is perceived as an ordeal to endure. If you've been sedentary all your life, you tend to accept the lethargy and ever diminishing physical capabilities as just a part of life and the aging process.