I'm interested in a hike like Kilimanjaro but have never hiked for more than a few hours and never camped overnight. The notes in the link say that while its not a technical trek, "previous hiking or trekking experience is essential." I've been an athlete all my life. I'm not in my best shape but am currently exercising regularly and practice jui jitsu. I'm wondering where to start and assess my preparedness for an 11 day hike. Not sure if its more conditioning or cardio or both. Thanks!
"The longest journey begins with a single step." Get out and hike. Begin with easy day hikes and ramp up from there. Find companions who can introduce you to multi day excursions. There is a lot more to it than just being in good physical condition, though that is a definite requirement. you should strive to know what gear and equipment,including clothing, works for you, and some sense of what your limits are..
Sounds like you are in ok health. Assuming you are in the US and want an easy introduction I would start with a guided backpacking trip in the US. REI offer some of these but there are other companies that do this.
If you are unsure I would pick something in the mid strenuous range as a first outing.
These are much cheaper and easier to do than an Africa trip and will give you a good idea as to whether you like backpacking and allow you to start assembling the gear you need. That is what I did although not through REI. I should say I've not done Kilimanjaro but I have since taken my own trips in the US.
"Guided" means that many of the logistical issues that get in the way of starting are removed like permits and food planning. Plus you get a knowledgeable guide to teach you the basics. Make sure to tip them generously since it's not a lucrative calling.
There is no direct equivalent to the Mount Kilimanjaro Climb in the US because its ~19,300 feet but you can climb to 14000+ feet in the Rockies and the Sierras and this provides a good test of how you might react to the altitude. REI doesn't appear to have specific trips for those areas so you might want to plan that as a follow on trip once you get grounded by doing a guided trip elsewhere.
Cardio, core and endurance are all important. You can get these variously from hiking hills or hiking with a loaded backpack or jogging (eg train to run a 10K). Lunges with a gallon milk jug in each hand are a commonly recommended exercise for backpacking if you must. But really if you are in ok health the most important thing is having footwear that works.
Both @OldGuyot and @hikermor have really good suggestions for overall conditioning and planning for the hiking and altitude aspects of your potential journey. I noticed that you mention that you've never camped overnight. Getting a good night's rest is paramount in an extended hiking excursion. While it seems very basic, I would suggest going to a local park or campground, for a few overnighter and/or weekend camp-outs so as to familiarize yourself with the feeling of sleeping in those conditions. You want to become comfortable being in the outdoors overnight, so it's second-nature when you are on the trail.
Good luck and enjoy!