Kilimanjaro asks for endurance rather than power. You are walking for five to nine days, six to eight hours most days, and on summit night far longer — all on a gentle-to-moderate gradient at altitude. Train for time on your feet, not for speed.
If you already hike regularly, eight weeks of focused preparation is enough. If you are starting from a low base, give yourself three months and build gradually to avoid injury.
Nothing prepares you for walking like walking. Aim for one long hike a week on hilly ground, building from 2–3 hours to 6–7 hours by the end, carrying the daypack you will actually use — loaded to 5–7 kg. Add a shorter midweek walk with hills.
Two or three sessions a week of running, cycling, swimming or stair-climbing to build your aerobic base. Add simple leg strength — squats, lunges, step-ups, calf raises — and core work, twice a week. Downhill is where knees suffer on Kilimanjaro, so include some descending in your hikes and don't skip the poles.
You cannot replicate 5,895 m at home, and altitude tents make little practical difference. The best "altitude training" is choosing a long route and, ideally, climbing Mount Meru a few days before. If you live near mountains, spending time above 2,500–3,000 m in the month beforehand helps.
Taper right down — a couple of easy walks, then rest. Arrive well slept and not nursing a niggle. No technical skills are needed; fitness, the right number of days and a slow pace are what get people to the top.