Do you get tired of sit-ups and crunchies? Try skipping to strengthen your core.
Skipping helps to strengthen opposing sets of muscles rather than concentrating on just one set at a time which can lead to injury in the set which is not being exercised. Back muscles and abdominal muscles both need to be worked in order to be properly balanced.
Try skipping for six or seven minutes at a time and you will find that your running becomes easier and you work the muscles involved.
This video shows 23 different steps that you can try while skipping. Don’t try them all at once. Concentrate on one per session until you can do them all and then perhaps you can perform two or three per session.