Your legs will be doing a huge number of revolutions; if you pedal at 75 revolutions per minute, your legs will be doing 4500 revolutions per hour. All of this will impact on your ankles, knees, hips and lower back
It is crucial that your saddle is at the correct height and set back. An incorrect set up may lead to back ache, stiff muscles, and sore hips/ knees. Any changes to your bike set up should be done in increments over time so the body can adjust gradually to the new position.
Finally, always carry a spare inner tube, tyre levers and pump and make sure that you know how to change a flat. Failing that, ride with a friend who does!