Some fitness tips and running tips/training schedule please?

Someone I know is really interested in fitness. In the fall they are going to go into cross country (it is now only 3 weeks away). They will have to run 5km. They already run fairly often typically 5km in distance like 1 to 3 times a week, but were wondering about how much they should train if they want to do really well in cross country. They want to be able to run their absolute fastest. They were also wondering about some possible fitness or running tips. Like hill training? or diet? or how to get strong stomach exercises? Anything would be of help!!! Oh and they need to get fast and in top physical shape in 3 weeks!!! Thanks!!

You should probably start doing some repeat miles and longer runs