On a bicycle, happiness is measured in a number of ways. For me, it's measured in speed and strength, and often the two come hand in hand.
I ride my bicycle at least four days a week as a form of prostration to the timeclock gods. It used to take thirty minutes and change to get where I need to go in the morning, but lately (sometimes due to enjoying my bed or hot shower for "five more minutes"), I've been rushing. Which is bad. However, I've made lemonade out of said lemon and have managed to train my body to cut out ten minutes of commute time. Which is actually pretty awesome.
During the process, I've found that my lower body is in a lot more good pain than normal; good muscle pain vs. bad muscle strain. Who needs a gym when you've got a bicycle, a commute, and the propensity to sleep in? I can't stress how important the benefit of a strong core and back is when it comes to riding long distances, or any distance in general, and I swear I will make a more concerted effort to leave the house earlier from here on in. But even still? No gym necessary - not for me.