My experience is that in the first 2-4 weeks, I found my stamina and endurance greatly reduced versus my pre-diet levels. I literally cut my work outs in half and really had to work to finish them

After that the level gradually increased to my pre-diet levels in maybe a 2-4 week span.

I was working out fairly hard -- 6-10 miles on the elliptical at 12-13 setting 7min/mile. Not the same effort as running at that pace but strenuous. For me it was near the edge of "I cannot carry on a conversion without getting out of breathe" effort or what I think of as my 75% level.