Experience from doing kickboxing for almost 3 years.
1. Generic 'Kickboxing" varies in style and each gym has their own "right" way. The form I learnt was heavily influenced by the fact that the teacher was a long term heavy weight TaeKwondo competitor, he still taught taekwondo but also competed in kickboxing and later boxing. Switching club showed the difference - their form was lighter, more about speed rather than power and actually sacrificed some 'style form' for speed.
2. You will be spending most of your evenings down the dojo - for the hour beginner lessons initially, then for the longer intermediate lessons & sparring, you'll then start using the beginner lessons as exercise and to begin helping/teaching, then for the advanced - with beginner/intermediate for teaching.
Initially it was sun, tues, thurs then it became sun, mon, tues, wed, thurs. In the beginners sessions, I used to be front right (most senior student) if a blackbelt didn't attend, acted as demo punch bag and was partnered with the off-the-street trial lesson bruisers that thought they were 'hard' and appropriately punched the pad as hard as they could.. then it was my turn - showed the form, showed the speed with control and then warned them that next punches put that together to develop power (I used about 1/2). The looks on their faces being on the padded receiving end
Also I'd get partnered with the beginner girls due to the control, however I remember getting my ass handed to me during sparring by a 2nd Dan when I accidentally made contact with her butt a bit harder than normal sparring should.
3. You will get injured at higher levels - as you spar internally initially it's all straight forward but as you get to brown/black you will be sparring with other club members. Then something stupid happens with those that just start club sparring - people forget they're sparring a human, they see the club and their own clan members as human and you as meat. This subsides as you start fighting the more advanced members.
If you job involves meeting people, clients etc you may find that you need to explain the odd bruise, broken toe or injuries.
4. 3 minutes - that the average length of sparring round, so you will find that your fitness becomes rooted in explosive short bursts of high intensity activity with the lessons also providing shortish periods of activity. So you will need to couple it with a decent run or bike ride to ensure you have a longer, lower intensity fitness too. On the good side - you end up naturally getting a good work out better than any simple gym weights session. In fact the upper belts required the student to train in a manner for competition - including diet and overall fitness.