Anyone else tired after working out?

I am used to doing cardio for 45 minutes and weights, but since being on the MC, I just don't have the stamina. I can only last 25 - 30 minutes and when I walk my dog, my legs get tired.

This morning is my 9th day and I felt really good coming to work...until I worked out at lunch and now I just want to go to bed.
I read someone else's post that they were continuing to exercise 45 minutes....I just dont see how.