So you have gained one extra lesson, added a bit of work on to set cover and, presumably, are sharing the extra bit of marking for any big end of year exams.
Another department has been lucky to find someone for cover.
Many years back now I was a cover teacher for a while because it suited me. I may have planned a few lessons on a longer-term stint (I didn't have to - again, it suited me better than rocking up every day not knowing what I was doing), but I certainly didn't mark any work. Even during those times, the department told me how incredibly lucky they felt to have found a competent subject specialist. The calibre of supply staff these days is far worse from what I see now.
In the nicest way possible, you are looking at this the wrong way.
The reason you have gained a lesson is because you will have been under allocation beforehand - that means you have benefitted from extra time up until the point you were told to cover unless your school has reduced your PPA time to less than 10%.
You are jointly responsible for the results of any shared classes, so being fully responsible for setting decent cover works in your favour. And decent cover doesn't have to be arduous; it can and should be a case of "here's a text book, summarise the page and do these questions" for your own sake, that of the kids who will do the work regardless of what the rest of the class is up to and for the sake of the poor member of staff who may be out of their depth if asked to do a full-on lesson.
Most marking can be done in-class. It is a useful skill for the kids to learn how to navigate a mark scheme, so incorporate this as part of cover, get kids to swap their tests and peer-assess. Save for mocks, most end of topic tests are bullshit anyway and, at best, exam practice to prepare kids for the real thing, so their accuracy rarely matters. And by the time end of year mocks are rolling around you have gained time.
If you send photocopying to the reprographics/ admin team, your time should not be impacted by more than 10min a day by setting cover.
I'm not just speaking hypothetically here; I am currently responsible for doing exactly all that for 2 classes.