al78,
IMHO it sounds like blood pooling in your calves. It should wear off when you start moving around but it can be tender or feel stiff at first. On the bike and walking about the muscle contractions keep pumping the blood back up your legs. When you're idle it runs down. I sit for long periods at a desk and try to keep my legs moving by pressing down on the floor with my toes raising my heels. I also rotate my ankles left and right keeping my toes on the floor. Good job it's under the desk because I could look a right arse.
You could try compression socks too.
Elevation would be good too, can you hold cards with your toes?