Sometime, we do things we don't really want to do, but we know is good for us. For example. This morning, I went to school, 16.25 miles in 59:43. I was pretty tired that whole day, but by the time school was out, I was pretty much recharged. Unfortunately, there was a ride that I had to make in order to get back to my apartment. I could have easily called someone or gotten someone to pick me up and drop me off near my house... but I knew that the better choice was to cycle all the way back... which I did. 17.05 miles in 1:07:25. Now the way back had more hills, so I had a harder time, but I pushed on.
During those times, our body may feel weak, or mind may feel tired... but you can push on. It's doing the right thing. It's doing what truly is important. I needed that bike ride. I needed to push myself for over an hour to go 17 miles. Doing that made me push myself and become a better athlete.
Sucking it up at first makes you ask yourself why... but after you finish, you remember the reason.