Hi Jane,
I haven't felt this for a while, but when I first began therapy - maybe my first 3 sessions or so - I felt very physically tired afterward. They weren't tough in the sense that I was talking about hard things, but I had just never talked about myself to anyone. So, yes, it did kind of feel like running a marathon.
Now that I think about it, though, I remember that over the summer I went through a period where I had to take a nap after my sessions. I was lucky enough to have the time, because I definitely needed it. Those were also the sessions where I had more of a tendency to dissociate than I do now (but I dissociated some last week, and I was tired afterward).
Sorry you're feeling so tired. If you can, try to let your body rest. The naps I took not only refreshed me a bit physically, but it also cleared my head a bit so that I could process the session better. (((Jane)))