Whether you are climbing Kilimanjaro or trekking the Inca Trail, training for your adventure should become a daily routine.  Any trekking venture is within reach for most people with normal fitness levels, but the more physically fit and prepared you are, the more you're likely to enjoy the journey (instead of being totally focused on your sore legs and feet). You'll hardly ever hear a returning Kili climber say "I wish I had trained less for that."

Walk, its what you'll be doing.
Running, bicycling, swimming, elliptical trainers, and so on are great for training your aerobic fitness, but remember that on the trail you will be walking. By all means do the other activities, but be sure to include walking in your fitness routine too, as muscular fitness is very activity specific. Every December I remember this when my legs are sore, not from the biking and running that I've been doing as cross training, but from the return to skiing.

Also try to wear the hiking boots you'll be wearing on the trail. If you've got full leather boots, they'll require some breaking in before the climb. Walking in your boots will also help to toughen up your feet, and you'll learn if there are any spots that cause blisters or pain. You don't want to find out on day 2 of a 7 day Kilimanjaro climb that your new boots give you agonizing blisters!

Challenge yourself to walk whenever you can.
An hour long evening stroll through your neighborhood is good training, but a full day hike is better, and best of all is to do long walks on consecutive days. On a typical Kilimanjaro trek, you might be averaging about six hours a day of walking for seven days in a row, with one grueling summit day of 12+ hours. On the Inca Trail it will be less, but still be prepared for some 6-8 hour days.  Your training should include a few walks or hikes of at least 4-6 hours to know what you're in for.

We know it can be hard to fit full day or multi day hikes into a busy schedule, so try to make walking a part of your daily life. Try walking to work or school, and when you have errands to accomplish, ask yourself "can I walk there?" It may take longer than driving, but overall you'll save time (for example 1 hour walking to the store, vs. 20 minutes driving + a separate 1 hour training walk), plus you'll save money, gas and improve your carbon footprint.

The mall is good, but hills are better.
Remember how muscular training is very activity specific, and a summer of cycling doesn't prevent sore legs on the first day of skiing? That also applies to walking on the flats vs. walking up and down hills. If at all possible, you should include hikes in hilly terrain in your trekking fitness plan. Best of all is to do sustained climbs and descents on other (smaller) mountains, but I get by with walking laps up and down a steep section of a local hiking trail. Other climbers have trained by climbing the stairs in a multi-level condo building or office towers.

Don't forget that descending is as much a part of the trek as climbing. I actually found the descent from the summit to Millenium camp on Kilimanjaro (around 9,000 feet of descent in one day) to be the toughest part of the trek. Hiking poles are helpful for balance when descending, and to take a bit of the strain off your knees.

Remember, your trekking fitness training plan doesn't need to be complicated, walking lots is key, getting hills in is better.   Make sure you start at least a month before your trek (earlier than this is even better), as it will be too late if you wait til a week or two before departure to start training.