21 weeks babymoon walking too much?

Walking in the second trimester is generally quite safe, and some mums I know kept up high step counts well into late pregnancy without issues, but 20,000 steps daily is on the higher end, and that evening tightening you're describing sounds like it could be round ligament pain, which is your uterus muscles stretching as baby grows, and is pretty common at this stage. The key trade-off is that light cramping from exertion usually settles with rest, but if the tightening becomes rhythmic, more intense, or doesn't ease off after you sit down, that's worth taking more seriously. My friend was also quite active during her second trimester trip and her advice was to break the walking into shorter stretches with proper sit-down rest in between, rather than long continuous hours on your feet, which seemed to help with that end-of-day heaviness. So enjoying your babymoon at a slightly gentler pace, maybe mixing in more café stops, could give you the best of both worlds without pushing your body too hard each day. That said, since the cramping is happening consistently, it's really worth dropping a message to your gynae or doctor just to get their read on it for your specific pregnancy.