I know how you feel. A month ago, I had the pleasure of jamming with a band for the first time in my life, and until I got the offer, I had never practiced standing up. What I did though, was to slowly convert myself from sitting down to standing up while practicing. Eventually it started to feel just as normal as playing while sitting down, but it took some time. Just start easily. The iconic main-riff in Smoke On The Water is a great riff to practice playing when you're not used to playing while standing up, because it's easy to play. When you can play that, try challenging yourself a bit. But to begin with, it's more important to be able to play something easy while standing up.