We think the affected leaves look like a type of leaf spot disease, perhaps angular leaf spot or anthracnose, which are very common in our Maryland gardens. Pinch off the most affected ones.
Here is our page with the most common problems that cucumbers tend to have: 
https://extension.umd.edu/resource/key-common-problems-cucumbers/
Many of the issues can be remedied to some extent. Particularly for diseases, it's best to

• look for and plant disease-resistant varieties.
• Avoid getting leaves wet with water by using drip irrigation, a soaker hose, or careful watering technique.

• Do not work in plants when wet.
• Remove and destroy infected vines at the end of the season in small gardens.

Fungicides/sprays are not curative, which is why they need to be repeated regularly, and so we don't generally recommend them.
If you explore the link above you'll be able to learn more.

It is not too late to plant more- July 1 is ideal.
Here is our Growing Cucumbers page:
https://extension.umd.edu/resource/growing-cucumbers-home-garden/


Christine