There is a super-fit runner in the sewers who is eagerly awaiting challengers.
If you type
challenge when in his vicinity, he will give you a time frame and a destination. If you can make it to your destination in the time he specifies, you will earn some XP.
You can find the super-fit runner in the Cormallen sewers. From the entrance, move
south, south, south, south, south, west, west, south.
There is a well that opens up to the sewers two east from the runner that is immediately adjacent to the bank and post office. If he challenges you to go to either of those locations, you can use the well to get your time down.