Bike maintenance is extremely important. As with many other things, once one thing is going wrong, it is typically an invitation for more things to start to fail. If you ignore one small problem, the chance that the problem will grow is pretty large.
For example, if you notice that your chain is a little rusty, and that it could use some oil, do not wait! Treat your bike as you would want to be treated! I have had so many more quarrels with my bike that could have been avoided had I just treated the early warning signs rather than waiting until everything went wrong.
One huge problem of mine is that the quick release portion of the back wheel comes loose easily. Well, either it loosens up while I am riding, or someone is trying to steal that back wheel when I leave it outside in Boston. To be quite honest, I have a feeling that someone is trying to steal the wheel. They usually do not get very far, because they realize what a pain it is to get the wheel off of the gears and such; however, someone did get as far as to cut off my brakes. I have no idea why you would ever do that! What do you gain from doing that? Furthermore, there was no need to even cut off the brake in order to get to the wheel. In any matter, finding my bike in a position that is unsafe to ride, is a huge signal to me that I should fix my bike. The best solution would be to get a fixed set-up in place rather than the quick-release wheel.
Having a broken bike will impact your exercising. For example, without brakes, I hesitate to go at fast speeds. There is no way I can get a workout in without having brakes on my bike. Even having a flat wheel will decrease the efficiency of your workout. It is important to take care of your bike, otherwise you will find yourself in a position where you will have to walk the bike back home.
Take care of your bike so that you can take care of your fitness levels!