Everyone should be working on core stability by building up strength in the tummy. Sit ups alone won’t strengthen this area. A runner's stride pattern is controlled by the core stability muscles in the pelvis and abdomen. So, to keep form strong when you start getting tired you need strong core muscles.
Basic core exercises are plank, side plank and bridge. You should be able to do 3x60 seconds of these and to make them more difficult you can lift your foot or hand off the ground when holding the position.
Also, a combination of lower abdominal sit ups or trans abs exercises can be included in the programme.
If you have a strong core you will find you can hold form for longer even when you’re tired.
Get those mats out and start working!