First and foremost is nutrition. Without proper nutrition you can't expect your body to build itself properly. Muscle, and a lot of your body, is made of protein so you need to ensure you have quite a bit in your diet. On the flip side you don't want to over do it or you can start to overwork your kidneys (they filter your blood). A good way to get a good balance is to eat eggs for breakfast (scramble, poached etc), eat some form of meat at every meal and drink whole milk (or even gold top!). Avoid processed foods and high sugar foods as your body won't get proper nutrition from these. Buy your food from a market or farm shop too and you'll generally get a better quality of food. Eat food like this and you won't need protein shakes, we promise!
Next is exercise. Quite simply put you need to stop any long distance type exercise. The whole time you're competing or training in long distance you're asking for trouble when it comes to putting on muscle. You need to hit the weight rack. It doesn't matter how much you can lift because everyone has to start somewhere, so don't be intimidated or think you've got to load the bar. Every time you work out make sure you complete squats and deadlifts. If you want you can throw in some shoulder press and bench press too, but it's not that necessary. Squats and deadlift are some of the most effective exercises out there and your body will react to them by building lean mass. Try to get the weight right so you can lift 6-8 repetitions before you can't do anymore. Have plenty of rest then lift that again, then again. Your workout should be about half and hour to 45 minutes and you should do it 3 times a week. Mix it in with some interval sprints (uphill is even better) and you'll get your body in muscle building mode.
Lastly you need to look at your sleep and stress levels. Lower stress levels and increase sleep. High stress and low sleep will keep your body in a perpetual 'catabolic' state. That means your body is continually in a fat and muscle burning state. Reducing stress levels and increasing sleep levels will enable your body to kick itself into an 'anabolic' state which means it's building muscle instead.
This advice will work for both men and women. Men will generally be able to put wight and muscle on more easily due to higher testosterone levels. Women that follow this advice will put some muscle on, but won't look manly. It'll give you a toned body that you'll be proud of and also increase your health.
Follow our advice for a few months and you'll see a massive difference. Remember it's not a race. Anytime you're trying to change your body you have to have patience and be able to invest the right amount of time and effort.
Hopefully that's helpful and let us know if you need more specific advice!