feeling out of shape

It really sucks having the pool closed. I think I need the swimming to restore my body. Thursday I did an easy 40min ride on the rollers, and yesterday I did my first ride on the Serotta - 100km. After about 90km, I was getting tired and had to drop the speed. My neck and back were sore... but it was also windy so I was spending a lot of time in the drops. Everytime I ride I think I remember I need to work on core, yet I never do... I feel so out of shape...