Raising money for charity can be difficult, but rarely is it as hard as Alex Ouellet made it over the weekend. Ouellet, a 22-year-old engineer and ultra endurance athlete, did thousands of burpees to benefit the Family Reach Foundation. (What's a burpee, you say? It's a grueling form of calisthenics that involves squatting and standing, over and over.) Coached by Jeff Butterworth of Rx Strength Training in Somerville, Ouellet was bidding to make the Guinness Book of World Records for most burpees in 24 hours by doing more than 10,110. Alas, he did 3,525 burpees at the Cambridgeside Galleria, but still raised $90,000 for Family Reach, which helps families fighting cancer pay for food, rent, utilities, or medical care. "Doing over 3,500 burpees is hard, but having cancer is a lot harder," he said.