Henry,
There's a broad range of ages, body types, etc that make it. It's already been said, 5 miler, 12 mile, ruck and PT test are the prime measuring sticks, however, also make sure you can climb (everything- ropes, ladders, obstacles in general) and carry things. The key point is, you have to do all these things and BE ABLE TO DO THEM AGAIN TOMORROW, AND THE NEXT DAY- for however long selection is. A skinny marathon runner who puts up a 2:30 12 mile time is no good if he is too broken off to do anything tomorrow. I'd say being on the bigger side is an advantage, since strength is the most persistent physical attribute- meaning, it has a longer expiration date than endurance. You know you are big and strong enough to do it, so all you have to focus on is endurance and minimal maintenance work on your strength. There are hundreds of programs, resources, etc that will get you to where you need to be.
Don't worry about land nav, you get a little BS class in basic that will at least introduce you to the principals. In selection you get a day long block of instruction and then get led around by the instructors so you can see how it looks- it's more than enough to get you through.