I can tell you that one of the most important factor for improving running and duathlon speed performance is on the nutrition(if rest and workout hours are already properly addressed). Below are some of the most important foods that are tested to improve performance:
1.) Rice in your diet
Rice is rich in carbohydrates and the most important fuel for speed. Typically if you are well-conditioned and well-trained athlete, most of your fuel will be derived from fats (especially for endurance and long runs >10Km). However if you are running at your aerobic speed (70% to 80%) you are still burning some moderate amount of carbohydrates. Rice should be part in your meal so that you can have a stable carbohydrate intake. In Philippines, rice is a standard part of any daily meal (breakfast, lunch and dinner). You can buy rice in public market at around 40 pesos per kilo.
2.) Chicken liver
Chicken liver is one of the most important sources of iron. Iron is a very important mineral for fat burning. If you want to become a elite athlete, you should have a a stable iron supply in your diet. Eating chicken liver at least once to twice a week can solve the problem. If you want to eat more, you need to consult with your doctor. Although there are other sources of iron, this seems to be one of the most delicious alternative.
Iron helps the formation of hemoglobin, the red blood cells in your body that transports oxygen to your running muscles. If you are lacking with hemoglobin, then you will have a limited amount of red blood cells during any workout – this means you cannot burn fat effectively and you will have a hard time improving your maximum aerobic function (MAF) speed.
One evidence of lacking iron during workout is that you will have a hard time breathing even though you sleep and eat well (but lack in iron minerals in your diet). The reason of this difficult breathing is that your lungs needs to work very hard to compensate for the lack of red blood cells circulating in your body. Your heart rate will increase also. Chicken liver is very cheap but a very useful source of iron.
When you are doing running workout, you will stress your bones. In the long term, running increases the risk of early stress fractures. The solution is to have a stable calcium supply to your body. One of the best sources of calcium is milk. Any milk will do, but I recommend those milk that are proactive in preventing osteoporosis such as Anlene. I recommend you should drink at least one to two full glasses of milk on a daily basis. Calcium is an important bone mineral which improves bone density and bone growth. Your bones will become stronger with your workouts instead of wearing out.
4.) Fried chicken or any pork and beef
Cooked meats are good source of protein. Protein is very important for building muscle and repairing damaged muscle from your running workout. For example if you do a long run, you will commonly observed that your legs are tired and heavy. It is because the muscle mass gets hammered badly; the solution is to have proteins in your diet on a daily basis. Also since foods are salted, it is a good source of sodium which is an important electrolyte in your body.
5.) Fruits – Banana and Orange
Do not ever ignore fruits in your daily diet. For example, banana is rich in potassium which is also an important electrolyte. Orange is rich in vitamin C which will improve your immune system.