Most people prepare for Kilimanjaro climb with fitness training. While getting reasonably fit makes sense, the gym work outs or sprinting up flights of stair etc. will not prepare your body for the demands of a Kilimanjaro climb.
You need to get your body used to walking for several hours in uneven country, for several days. But any fitness training beyond that will not increase your CHANCES to reach the summit.
It’s the altitude that will get you, not your lack of fitness.
So if you can, expose your body to some altitude before you tackle Kilimanjaro:
If you are living somewhere near mountains, climb them! If there is a CHANCE to overnight at higher altitude, do it. (Note that for this to make a difference it needs to happen right before your Kili climb.)
Some people do acclimatization treks on Mt. Meru before they climb Kilimanjaro. We have been doing and can recommend it, but only for people with some previous trekking experience. Otherwise it may backfire… Read more about it here: Meru and Kilimanjaro.
There are other options: some operators offer cultural tours in the Kilimanjaro foothills, there are walking safaris in the crater highlands…
The Ngorongoro crater rim is over 2200 metres high and even the crater floor is at 1700 metres. If you think of doing a safari while in Tanzania, why not plan it so you can spend a night or two on the crater rim before transferring to Kili?
Look at where you will be spending the night(s) before your climb. Some agencies will put you up in Moshi, some in Marangu. Moshi lies at 890 m, Marangu at 1800 m…
No matter where you will be staying, definitely fly in a couple of days early!
Give your body time to adjust to the different climate, the food, to recover from the strains of a long haul flight and to get over the jet lag if you came from a different time zone.
Arriving early can improve your CHANCES of reaching the summit by five percent or more.