I'm over 50. How can I compete with these kids?

You don't have to. You don't have to set a record on a ropes course to get a job. You already did that. What you need to be able to do is show your commitment to the job and your understanding of the work environment. You have a work history that has to be cut back while the "kids" are padding theirs. Experienced construction employers can see the difference, and you can work toward showing them.