Fasting can be very beneficial for your body to "clean house", A lot of people in western culture are very weary of fasting. But ancient cultures have been fasting for thousands of years.
What it all comes down to is "the proof is in the pudding". I have fasted and know many people who have fasted therefore I know from my personal experience that it works. Many friends of mine have done the master cleanse (lemonade diet) by Stanley Burroughs and one friend of mine said after the third day you are not even thinking about food and have soo much energy. The reason for the energy is your body has all of that energy that it uses to digest solid food free to use. But on a one day fast you may not feel this way.
I would recommend a juice fast the first time because it is much easier on your system. Plus the juice helps your body deal easier with the toxins being released from your organs etc.
Of course before you do your fast read a bunch of literature on it and plan out your "battle plan" Therefore you know what to expect.
One great benefit of fasting is time seems to slow down and a more humble attitude is cultivated since you are controlling your body's desires. And when you do eat again you are much more grateful for your food that comes in.
And when done your fast you will be very sensitive to your food coming in. Try to eat good food as opposed to lots of food and you will have more energy because you are giving your body more efficient fuel.
As for training try not to do too much strenuous activity on your first fast take this time to slow things down, do some stretches, go for walks in nature take this time to cleans and rejuvenate. Also don't engage in ual during fasting it will suck your vital energy out of you.
You can even go to a naturopathic doctor or holistic nutritionists and do the fast with them for safety and guidance.
Hope I helped a bit.
All the best