In 20 years as a paramedic, I used a tourniquet once, and that was on a guy with an arm entrapped in machinery.
Roody's advice is best-call for help. Heat and orthopedic injuries, as well as individual specific health problems will be most common, so a basic first aid course will teach you all you really need to know.
When my wife and I rode on a heavily used park trail, we carried a little bit of gear, but the only thing that cannot be improvised is a pocket mask for rescue breathing during CPR.