A few weeks ago I challenged the kids to run 2 miles -- from our house to Kim's. We started a series of training runs trying to build their stamina. Although we had not even gone halfway on those training runs, the kids told me that they were ready to run the whole 2 miles. And they were. They all made it without stopping. Everyone but Jake had to battle through some ache or pain, but they did it. I was impressed.
I promised the kids an extra large shave ice from Ice Age if they could do it. (I know it is somewhat confused to promise a reward that has more calories than we burned in all of the runs combined, but they could actually use the calories.) It was great motivation.