Achieving a reasonable degree of physical fitness should be a goal in your preparation. Being in good shape will increase your chances of having a safe climb, successful summit, and enjoyable experience.
The main reason that climbers fail to reach the summit is due to the inability to acclimatize to the high altitude quickly enough. Short of going to high altitude, there is little that one can do to pre-acclimatize before the climb. Being physically fit does not guarantee that climbers can overcome altitude issues, but it does reduce the strenuousness of the climb on the body, which in turn, makes acclimatization more likely.
Ideally, your training should simulate actual conditions encountered on Kilimanjaro. Performing day hikes on local trails are the recommended form of training. The trails should include uphill and downhill sections, and you should wear the clothing, boots and daypack (weighted) that you intend to climb in. Try to hike a few times a week, with a mixture of short hikes and hikes that last for several hours. Your hikes on the mountain will on average be between four to six hours but can be as little as two (easy days) and as high as 14 hours (summit day).
If it is impractical for you to train outdoors, you may exercise at the local gym. The goals of the training program are to boost your aerobic fitness and to increase your endurance. The staple of your training should be walking on a stair climber machine, supplemented with weight training for your legs.
A minimum of three days a week, perhaps shorter sessions during the weekdays and longer sessions on the weekends, for three months, is suggested. Sometimes, try hiking on consecutive days. With proper training, you will develop the leg strength, endurance and confidence necessary for Kilimanjaro.
Here is a sample Kilimanjaro training program.
For those who cannot train at high altitude or want an added advantage, there are in home systems that may help you acclimatize to the high altitude prior to arriving in Tanzania. Usage of a high altitude system can reduce your symptoms of altitude sickness and increase your chances of reaching the top.
These high altitude training systems by Hypoxico can be rented for home use just prior to your trip to Kilimanjaro.