I know exactly what you're going through. I've been back at work for nearly 4 months. The first couple of weeks were hard, then I went from 30 to around 36-40 hours per week and it was just too much. I tried 5 days at shorter hours, but me being me, I would end up staying on til everyone else was finishing, so that didn't work. What I do now is work 4 days, with Wednesdays off to recover in the middle of the week, take any treatments or dr appts etc. That way I manage much better. I can even manage a longer day on the 4 days I work because I know I will get a good break in the middle.
Is that an option for you?
I didn't work for nearly 3 years. In the last 12 months I returned to work on a very gradual part time basis for four months, but that job didn't work out. Then I didn't work for 5 months, followed by 6 weeks of 5 days at 5 hours a day, which was wonderful.
Now, my job still exhausts me, but I'm getting to adjust to it a little more each week.
My best advice is to hang in there, see if you can shorten your days a bit while you're getting used to the 5 days. You will adjust after a while.
Congratulations for getting through your first week.