The Sunday of the marathon they signed up for came more quickly than Claire had anticipated and she was freaking out a little bit about not being able to finish it. Alex had to reassure her that she had followed the training regimen well so she should be fine.
Ethan had gone to Ray's house for the day since they hadn't known he would be staying with them and hadn't registered a spot for him in the race. To get him to go willingly she promised they would sign him up for the next one with them.
Alex wasn't worried about Claire's ability to finish the race. He was worried about slowing down to keep pace with her. He couldn't just run ahead and leave her behind in case she needed motivation to keep going every so often.