When you say "5kgs at first", what sort of time period were you thinking?
Typically weight loss will be large at the start, then the difference will be smaller as your body shifts its balance between fat and muscle mass.
A combination of exercise + diet will help you achieve and maintain a increased level of fitness (and faster/better results).
I start my legit effort to improve fitness/etc at the start of this year, and lost 3kgs in the first month. I only made slight changes in my diet, not as much as I probably should have - things like drinking more water and trying to make healthier food choices, especially when eating out with friends, etc. After 4.5 months, I'd lost 5kgs and a dress size. Then was on a 1 month hiatus because of exams, and started up again recently. It's been 6 months now and I've maintained a total weight loss of 7kgs, and can observe vast changes in muscle tone and general energy.
I know it doesn't sound like much at all, but the weight loss i've managed to achieve feels much more permanent than any dieting or anything else i've tried before, which i think is more important than just shedding kilos for the hell of it.
The program i maintained throughout the year so far (with a heavy uni timetable as well) was ~1 hour sessions, minimum 4 days a week (aim was 5-6), combining cardio, resistance and weight training, including 20-30min weight sessions 2-3 times a week. Interspersed between that was 1-2 weekly step classes and personal training sessions when i felt like it.
Each session would consist of any of the following to fill up that one hour:
- 20mins step climber, 5.5km/hr on max tread
- 20mins bike
- 20mins x-trainer, 120 strides/min minimum
- 20mins on treadmill, max jog at 10-11km/hr
- 10mins rowing machine, 2000m
- 25mins on the weights circuit
- 15mins on chin up/etc + leg press machines
For cardio, i'd approach each session with an approximate goal for how many calories i wanted to burn. atm i'm looking at 300 calories minimum
Not sure if that's what you're looking for, but it worked for me. I guess increasing session length and a stricter diet would give you better results than i got. I still eat unhealthy food and maybe i missed my class for this week >_>, but this is something i found manageable.
hope this helps and good luck