I have done a lot of racing in everything from 800m to 50+mile mountain ultras, and each distance has it's own special kind of pain. Here's what I feel:
800m - Lungs burning, taste of blood in your mouth. Feels like you strapped on concrete boots after 600m. You are at 110% of your limits for the last part of the race, but not necessarily "painful" in the classic sense. Still, might be one of the hardest distances.
5K - Lungs burning, legs tired in the last mile, but not overly "painful". You are breathing at your max rate for the last half mile which is stressful, but is quite manageable if you paced yourself correctly.
1/2 marathon - Easiest distance for me. Feels like a quick tempo run and lungs don't really come into play. Legs will start to get heavy and you have to fight to keep up your tempo+power over the last few miles.
marathon - Breathing is moderately easy, the first 16-20 miles should be a breeze if you've pace properly. After that, legs get increasingly heavy, and vaguely painful the further you go. If you've gone out too fast, you are susceptible to sudden cramping and loss of power. You are in for an hour long painfest at that point.
mountain ultras - On the flats, you are running at your "easy day" pace. Breathing is easy until you hit serious (1000ft+ hills) at which point you may be hiking, but lungs and legs are burning. If you've paced correctly, you will notice more difficult holding pace after 20-30 miles, but if you've gone out too fast or messed up your race nutrition, it will feel like bonking in a marathon, and you still have 25+ miles to go. Terrible leg pain, stomach nausea, inability to lift legs over rocks and roots, etc.
The marathon is the race that causes the most pain in the days following due to the volume of trashing in concert with the relatively fast pace (about 1 min/mile slower than a 5K for a hobby jogger like myself). A poorly executed ultra is the most painful race overall due to the many hours that you may have to suffer through a bonk. However, the slower pace mitigates the muscle damage, and I can usually get back to training a couple days after a race. The 800m is the most intense pain, but it only lasts 30 seconds or so.