Getting muscle definition (including the coveted 6-pack) requires reducing your body fat percentage. How that's done is generally two things - eating right and strength training (generally either lifting heavy or doing advanced bodyweight exercises).
Most sources I've found say that you do not need to run to achieve this. However, 3km isn't much, so it probably serves as a decent warm-up/cool-down (additionally, cardio is good for the heart). Whether you drop it or not will be up to you.
There are a number of programs out there that you can do to build strength. Pick one that appeals to you and that you can stick with ("the best program is the one you can stick to"). Here are some resources/options to get you started:
- New Rules of Lifting (weight lifting, book)
- Starting Strength (weight lifting, book)
- StrongLifts 5x5 (powerlifting, website; I, personally, like and recommend this one)
- You Are Your Own Gym (bodyweight strength training, book)
- Convict Conditioning (bodyweight strength training, book)
- Beast Skills (bodyweight strength training, website)
- Never Gymless (bodyweight strength training, book)
- Crossfit (as I understand it, High-Intensity Interval Training, with some weights; group program)
Most of these programs will have either a 3x/week plan (ie - full body, three days a week, with 1-2 rest days in between), or a rotating 5-6x/week (ie - upper body M-W-F, lower body T-Th-S, day off completely on Sunday). Rest days are important! You're "breaking down" your muscles when you work out, and building them bigger and stronger when resting (the act of lifting creates microtears, resting rebuilds those and makes those parts stronger).
Keep in mind, too, that getting the 6-pack is considered "80% diet." That is, what you eat matters, especially once you're body fat percentage gets down close to and into the single digits.
- Be careful of things like lemonade, as they tend to have a lot of sugar in them. The same goes with Gatorade and whatnot. If you're working out to the point where you need electrolyte replacement, consider making your own.
- Eat clean. The more whole foods you eat, the more nutrients you'll get. If you can do things like get your animal products from free-range, grass fed animals, even better (beef from grass fed cows has more Omega-3s and nutrients all around).
- Experiment with your diet. Some people can't tolerate gluten (wheat/cereal products), and gluten intolerance can lead to stomach bloat, even if not other symptoms occur. It might be worth cutting out grain-based carbs to see how you react, especially if your BF% is low and you're not getting definition. The Paleo diet can be a good guide for this, even if you don't follow it completely.
- Eat lots of protein. I've seen it vary somewhat depending on source, but the consensus I've seen is 1g per pound of body weight.
- Don't be afraid of fats. Dietary fat does not directly make you fat. Eating more calories than you burn is what makes you fat. The macronutrient numbers I've seen are generally around a 30/30/30 split of carbs, fats, and protein, which adjustments generally being in favor of fats and proteins, depending on individual needs.
- Don't get hung up on the scale. Use it to help determine your body fat percentage, if necessary, but ignore the scale otherwise. Strength training usually results in gaining weight, but lowering BF%, so BF% is what matters. Body fat percentage measurements can be hit or miss, in general, but according to this article, a cheap and easy way is to measure yourself. I personally use this calculator to get an estimate and keep track. Like the scale, though, don't get hung up on the individual numbers. Monitor the trend to make sure you're going in the right direction.
- Track your food. There is often a big difference between what you think you're eating and what you're actually eating, both in amount and in nutrition content. Sign up at a tracker like MyFitnessPal or Livestrong's Daily Plate to make sure you're on track.