When you can't keep up with your fellow marathon runners, you can say that you lag behind them.
The word lag describes a kind of slowness or delay. As a noun, it means a slowing: "The coach was disappointed by the lag in her swimmers' best times that day." When you lag behind someone, you fall back or don't measure up. A student who struggles with math, for example, might say that his grades lag behind his best friend's. In the 1550s, lag meant "last person."