800m is worse than a 5k. There is a reason why they have road races and "5k fun runs". They don't have "800m fun runs".
Here is my feeling during the 800m
First 100m you get out fast, going really close to your 400m effort. You keep it up the second 100 to keep your position. You don't feel terrible yet, but I always have a feeling in my stomach like I know it is going to suck. You go through pretty fast, then it is the easiest 200m. The last 200m of the first lap you are cruising at a fast pace, but because of your training you can run this 28 second 200m and not feel too bad.
Now the real torture begins. It always seems like there is no fatigue the first lap and it all comes on you on the second. You have to pick it up to keep up with the competition, you begin to dig deep from 400m-500m. You are now on the backstraight, so far left to go is what you think. I become discouraged and try to keep positive here. Then there is 200m left. At this point I have lost the bounce in my step and don't feel like I have explosive power anymore. I try to push to beat out the competitors.
At this point you are tired and have some good lactic acid buildup and your muscles begin to feel tight. 100m left, so close yet so far. You are now basically going all out. Hopefully you get an adrenaline boost to bring you in. You try to focus on your form and lift knees and use all the power you can. You gut it out to the finish and are finally done. You begin to have blurry vision and are lightheaded. More crap happens but I am done.
Also I just want to add why the 800m is much worse than the 5k. The 800m has evolved into a race that is no longer distance but now a long sprint. More and more sprint types are having success, because that lactic acid buildup is what they are used to. Also why are there 5k runners on here talking about feeling lactic acid in a 5k? You realize lactic acid comes from anaerobic work (sprinting) not aerobic (distance running).
At any rate, the 5k is more relaxed and comfortable throughout the race. You don't question if you have enough speed and are burning yourself out. Also if someone goes out fast in a 5k you have much longer to cactch up and can do so slowly. In the 800m you react quickly and must stay within reach, which may mean running the first lap faster than you want. To me the 1600 is the most natural running pace, running one lap of it is not too slow, not too fast. 800m is a little too fast. 5k is too slow. But we all know slower is easier than faster.