Every year hundreds of budding new triathletes go out and purchase a new bike, or beg-borrow-steal one from a friend. And every year many of these same athletes end up with an injury from training on a bike that does not fit them.
Sure, riding a bike is a non-impact sport in general, but if you have a bike that is too big, too small, or just isn’t ‘right’ for you, you may end up dealing with injuries for months on end. Most people will think it’s the running that is causing their knees to lock up, their Achilles to become inflamed, or their low back to be in constant pain.
This is why it is very important that no matter what you do when you get a bike (whether it is bought or borrowed) is to make sure you go to someone who has at least been trained to fit bikes to your physiology. Because there is nothing worse then either being in pain while riding, or becoming injured from your riding.