The New York Times profiles champion ultramarathoner Scott Jurek, who runs on the order of 140 miles a week training for races that are often 100 miles or more, sometimes through deserts or frozen wastelands or up and down mountains. Jurek is a vegan, consuming 5,000 to 8,000 calories of plant-based nutrition a day.
And Jurek is not alone as an elite vegan endurance athlete. There's Rich Roll, one of Men’s Fitness Magazine's 2009 “25 Fittest Guys in the World”, is a top Ultraman competitor. Ultraman is a "double Ironman" three day triathlon, with a 6.2 mile ocean swim followed by a 90 mile cross-country cycling race, a 170 mile cycling race, and then on the final day a 52 mile double marathon.
Or there's Brendan Brazier, a professional Ironman triathlete and two-time Canadian 50km Ultra Marathon Champion.
Or Ruth Heidrich, vegan for 24 years, holder of three world fitness records in her age group, six-time Ironman triathlon finisher, and holder of more than 900 gold medals for distances ranging from 100 meters dashes to ultramarathons, who credits going vegan with sending her breast cancer into remission. (This is certainly a controversial claim, and I am not suggesting that anyone discontinue medical treatment.)
If you think that a vegan diet can't give you the energy you need, I suggest you talk to these folks -- if you can catch them!