There are all kinds of levels of fitness out on the trails, and there are many different difficulty levels on hikes. If you have all day set aside for a hike, then you can do just about any level of hike out there. The important thing to remember is to pace yourself, don't start out too quickly if the trail is going to be steep and long.

Are you doing consistent cardio? If so, there are hardly any day hikes that will be a challenge for you to complete. You will most likely pass a few people on the trail, but if you aren't used to the uneven paths, then prepare to be a little sore the next day. Continue your cardio and throw in a hike every few weeks, and this soreness will soon be in the past.

Lifting weights in the gym? You will probably still be sore after your hike believe it or not. Cardio could be an issue, though you will have no problem getting up the climb when the going gets tough.

Are you doing Crossfit or another high intensity training? I can say for sure that you will not have a problem hiking at all. A lot of the everyday workouts these types of programs have are exactly what many articles on this subject recommend. Bring on the switchbacks!

Are you sitting on the couch eating ice cream? Well...if its just this once! Realistically, if you have been athletic in your past and have fallen away from that, then hiking can be a great way to get back in shape. Please be a little wary and start on some shorter climbs, but there is no reason to "train".

Ultimately, as my wife says about many things, "If you have to ask....."

There is something to be said about a multiple day backpacking trip, especially if you are carrying a heavy pack. Some backpackers go ultra light and get away with packs a lot less than twenty pounds, and then there are those that take an overweighted pack to set up a penthouse on the top of a ridge. If you have packed too many amenities, then yes, you probably should have trained. Sometimes the hardest part for carrying a heavier pack for consecutive days is...the next day!

Here is a link to a write up on how to tell the steepness of a trail!