Avoid prolonged poor or awkward postures. Static load on the muscles will lead to early fatigue. Just because you are doing an activity you love does not make you immune to overuse injury. Many people on campus love to get dirt under their nails and hit their gardens as soon as the weather allows. That's fine, but just be smart about how you pace yourself, otherwise that ache in your elbow that hasn't gone away by Wednesday will severely impact your ability to be out there again the following weekend.
Gardening is hard on the hands and the back.
Mountain biking can stress hands, wrists, shoulders, and back. Rock climbing is incredibly tough on the hands and forearms. Don't stop doing your favorite activities because they may be tough on the body. Just be wise about how you perform them.