You have two options: Either don’t ever take three weeks off from your trainer, OR, if you have done that, never ever ever go back.
With my travel I ended up going about a month between sessions with my trainer. I’d been hitting the gym some though, mostly cardio, and was feeling pretty good. I was going faster, longer, on the treadmill than ever before, and my muscle mass still felt pretty solid. So I thought I was in for a mostly normal session.
I just wanted to die. I could barely do anything we usually do, and ran out of steam far quicker than usual. I also hurt more today than usual. So, bad idea though it may be, I’m headed back tomorrow for an extra session to try to kick my butt back into training gear. Wish me luck.