If it's that "oooo, I can do this but it's really hard to do" feeling, as opposed to actual pain which might indicate an injury, you can feel free to slow it down, take it back, and repeat weeks until you're more ready for them.
Nobody said you HAD to finish a 12 week program in 12 weeks. If you take 15 by repeating a few weeks, that's fine. You'll still get there!
Deb, in New Zealand