If something is a hardship, it causes suffering or unpleasantness. After all the hardship you endured while training for the marathon, you really hope that you’ll do well — or at least finish!
The word hard in hardship is a tip-off to its meaning: something that’s a hardship is hard. It could be hard work that you do for a larger goal — like training for the marathon — or it could be a situation that’s hard to endure. If you lose your job, you could end up experiencing financial hardship. Sometimes a hardship is something that just happens, like the hardship people suffer following a disaster.