let's remember the basics to keep going: End-Of-Race Form Tips For Ultrarunners (from irunfar.com)

Listening to your body (rhythm, breathing, and posture) rather than music is I find crucial not only to avoid any potential injury, but to enjoy the running with improvement (faster, stronger, etc.).

I find the following info useful which I try to remember and practice:

Get Tall
The first thing to go for many fatigued ultrarunners is posture. Mounting fatigue, as well as heavy hydration packs, can cause a forward-slumped trunk. Besides looking slouchy, there are multiple consequences to poor posture:

• Compromised breathing – A flexed trunk restricts ribcage motion, making it difficult to get a full breath. Try it. Take the deepest possible breath standing tall and with your back slightly arched. Then take another with your back slumped and shoulders rounded. Getting a full breath is integral in mitigating hard efforts, especially on inclines and at altitude when oxygen is at a premium. Taking a full breath also helps keep the heart rate in control.
• Decreased trunk mobility — The most efficient run stride has a nominal amount of trunk lengthening, shortening, and rotation. Slumped postures restricts these efficient motions.
• Decreased hip push-off — A flexed trunk causes a ‘flexed pelvis’, which limits how easily and strongly the hip can extend. This robs the stride of precious power.

How-to: Arch your back and stick your butt out–slightly. A good cue for what the head and neck should feel like is ‘long and tall’.

Lean Forward
Overstriding is, in my opinion, the number-one cause of inefficiency and injury in running. Anytime the foot lands in front of the body’s center of mass, this constitutes overstriding.

Trunk positioning is the primary factor in foot-strike efficiency. It’s also one of the first things to go during an ultramarathon. Fatigue makes it difficult to maintain the forward-trunk position, but trail factors can also influence forward trunk engagement: long descents and tiptoeing over technical terrain make it all too easy to lose forward momentum.

How-to: Simply leaning forward is an effective starting point toward reestablishing forward momentum. But the hip-hinge position is more powerful, and sustainable. A hip hinge feels like a balanced combination of ‘sticking your butt out’ and a chest-forward lean.

A good indicator of adequate forward engagement is the ability to see your feet landing beneath you. This ensures that you’re adequately forward, with minimal overstriding.

Pick up the Knees
Hip flexion goes pretty quickly with mounting fatigue in an ultramarathon, and a normally short, compact stride can become long and slow. Long strides create braking forces, which must be absorbed by the quads and calves, and stress out the hip flexors, which strain to swing a long and heavy leg.

How-to: Pick the knee straight up, using the abdominals. This straight-up motion has two effects: one, a straight-upward leg and foot lands straight beneath, limiting overstriding. Two, a strong, upward hip drive on one leg automatically enhances push-off on the other. Win-win.

Swing the Arms
One of the most overlooked, under-used form strategies in ultrarunning is arm swing. Hydration and fueling are huge factors in deficient arm swing: ultrarunners tend to be using their hands far more than conventional runners, carrying water bottles and handling gels and other food.

How-to: Swing the arms strong, small, and quick. Efficient arm swing is strong but compact and fast. Aim to drive elbows behind and slightly downward. This creates the same effect at the hips: enhanced forward drive and rearward push-off. Lastly, get the shoulder blades involved: shoulder blades should ‘slide’ down and back, leading the arms. This action also helps reinforce tall trunk posture.

Refer to this post on arm swing for helpful exercises and drills.

Conclusion

Fatigue is inevitable in ultras, but slowing down is optional. Use your head to keep your legs moving. Best of all, a form-focused approach at the end of a race is a great way to keep your brain focused on something other than the pain!