something similar happened to me during a 10k race at worksop few weeks ago. was feeling tired b4 race (had bit more alchohol night b4 than should have- very big mistake) its was so hot, the 1st 5k was awful i jus couldn't run practically walked all way, even thought of dropping out of race! then hit an inpromptu water station provided kindly by a local resident, drank cup water poured another over head ,then hit actual water station did same then i began to run like never b4 ran rest of race without stopping and felt amazing. sadly time was 1hr 6mins my worse 10k ever . but cant get my head round where that spurt of energy came from, must av bin something in the water!