I actually enjoy the planning of hiking trips and treks nearly as much as the hike itself. Thinking about where I'll go, what I'll eat, the supplies I'll need, and what I'll see helps get through boring stretches at work and commuting. Planning is kind of like virtually taking the hike and its fun to see how close I imagined it would be to how it really is.

When planning a hiking trip, you can't get too detailed. It's fine to calculate right down to the weight of your bandanna or the exact minute you need to stop for lunch. But, you do need to remain flexible at all times and be prepared to shift your plans as needed.
These main planning sections are good things to consider.

Take a Shot

Consider your current abilities from your recent training to determine how much of a hike you can handle - be realistic. Decide where you want to go - the coast, mountains, nearby forest, ... wherever you want to explore. Get a rough idea of how many hours you want to hike, how far you can go, what hiking supply load you need, and then use maps and guidebooks to find a trail that matches your desires and abilities.

Figuring out how far you can hike in a certain amount of time is a good exercise. Or, figuring how long it will take to hike a certain trail. The actual results will depend on your shape, the trail condition, elevation, weather, and lots of other little things. But, in general, you can count on 2 miles per hour on flat land. Reduce that a bit for every 1000 feet above your home elevation due to reduced oxygen. To the total time, add 1/2 hour for every 1000 foot elevation change due to slow climbing.
Here's a simple calculator that will give you a crude estimate - enter what you want and the rest will be calculated:

Click Here to use Calculator on your Site!

Tell a Friend

Find a hiking buddy to go with you. Hiking alone is not safe. Once you are an old pro and have been through some rough weather, difficult terrain, a few accidents and missed turns, then you can think of hiking alone. Until then, take a buddy along. Someone with more experience than you will be a great way to learn new tips.

Discuss your plans with your friend and make sure everyone understands how far you want to go, how fast you want to hike, and what you want to see on the hiking trip. Having expectations synchronized will make the adventure fun and fair with fewer surprises.

Check Terrain

Use a detailed topographic map to understand the difficulty of your hike. It may just be a 6 mile loop on the map, but that may be flat or include 4000 feet of elevation change.
Learn how to read a map before going hiking. When you are in the field, your map and compass will be your most important tools to stay on track. Also make sure your map is current - trails change, magnetic declination changes, areas open and close.
Checking with the agency responsible for managing your planned hike area is a good idea.

The elevation you gain and lose while hiking will have a definite effect on how fast you hike and how much ground you cover. Hiking up a steep grade will slow you way down, forcing your muscles to work much harder. Hiking a downgrade will be easier on muscles but much harder on joints.
 

Creating a hike elevation profile will give you a good idea on where the more difficult stretches are in your planned hike. Convert the information on your topographic map into a chart of elevation versus distance. In this example, a climb to the top of a peak and then the return on the same trail is diagrammed. It is fairly consistent with just one short stretch halfway up that is fairly flat.

Take into account the time of year when checking the terrain. A dry, dusty trail in July may be a muddy mess in May. You may be able to hike across a frozen marsh in January, but go around it in June. Also remember that higher altitude means cooler temperatures and hypothermia is a real possibility anywhere below 60F degrees. If your hike takes you over 9000 feet, read more about high altitude hiking.

Check Weather

Everyone knows that weathermen are seldom correct. Even with high-tech gear, predicting the chaotic nature of weather is not possible. A beautiful, sunny day can turn to life-threatening rain in an hour. You have to be prepared for the worst probable weather and its consequences. The word 'probable' is key - June in Minnesota will not require snowgear, but will require raingear. You need to consider what the worst effects will be from weather for which you did not prepare and then decide if it is worth the risk. For example, hiking in Death Valley in April has a miniscule chance for any precipitation, but an average daily high of 90F and a record of 120F. So, the consequences of no raingear are much less than not taking a hat and lots of water.

Elevation and weather are closely related. In general, every 1000 feet in elevation means a drop of 5F degrees in air temperature. Also, the higher you go the faster and more severe weather changes with big drops in temperature occurring quickly and higher winds in general.

A sudden rain passing through may just cause you to seek shelter for 15 minutes. But, your travel time may be greatly reduced afterwards due to a deep layer of mud on the trail. Be ready to change your hike plans due to weather conditions. Read more about what to do in case of lightning.

While hiking on your trip, constantly stay in touch with nature around you. Just walking through and oohing and aaahing over the flowers and mountains is missing half the fun. Check out the clouds - are they building? are they picking up speed? are they white or dark? what does the horizon look like? Are birds still flying around? Is there a gentle breeze in the treetops or is the wind getting stronger? Keeping an eye on your surroundings is fun and important.

Check on Permits

Most national parks and wilderness areas require a backcountry permit. Often times these permits are free but if you are checked and have no permit, the fines can be very expensive. Permits are used to monitor visitor traffic, to limit use of certain spots, or to help keep hikers safe. From very restrictive permits that define your trails and campsites to general access permits, it depends on the management goals of the agency in charge.

Other areas may not require a permit, but its always a good idea to check in with a ranger. It's an opportunity to tell one more person where you plan on hiking and to check one last time on trail conditions and any special short-term regulations in the area.

Tell Another Friend

Always, that is ALWAYS, leave your hike itinerary with someone at home.
Make sure they know your route, start time, when you will be back, and when you will be contacting them. If you are not in contact with them as expected, they should have instructions on who to call to check - for example, the ranger station near the trailhead.