OP you really want to dot your i's and cross your t's, eh?
I definitely would add a hobby if it really is a part of who you are/what you do AND it's kind of unique. It'll give you a lot more to talk about during interviews (though, in my case, none of my interviewers brought it up and it made me kind of sad). Even though you're a unique person and there's no one like you in the world, blah, blah, blah, every medical school candidate does pretty much the same stuff in order to get into medical school. So if you like to rebuild classic cars or do woodworking or are active in your community baroque orchestra you should include those. As for something like running/hiking/etc. I would include it only if you can add something special about it. Like you run 8 marathons a year or you do the type of rockclimbing where you don't use equipment. Others might say include it anyway even if there's no unique aspect to your hobby.