My* big brother is a marathon/trialthon guy and has been since I was little and he loves it. It is alot and I do think at the start you will be dead tired but once your body gets used to it (it always takes him a couple of weeks) you get more energy. He just ran the New York Marathon this year and he trained hard for it and loved it. He ran 15 miles sat/sun and between 5-10 miles 3-4 times a week.
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I have never done it because running is horrible and I hate it but I always wanted to be able to!! So if you do keep us updated so we can live through you!!!!
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Good luck!!