We've heard this in the TV ad for some arthritis medication and it's very true. Your joints may, indeed, be very painful when you first try to move them but once you get them going, the lubricant within will start moving around and cushion the bones so they are not rubbing against one another. Once you're up and about, you can continue to keep going without any problem. Your body is very happy moving around all the time because the pain is minimized.
In life, it's the same. Procrastination or inertia keeps you from starting something - perhaps there's a habit you wish to change and you know exactly what you want to do but you just "never get around to doing it" (I've certainly been there, done that and have plenty of the t-shirts).
However, once you get that first step taken and do something once, even if it's just a part of what you planned, then it seems easy to keep doing it. Then you want to do it again. The next time, you've started to develop more "grooves" for the new habit and it's easier to start - you might actually even be excited to start doing it. You could even get to the point where it's hard for you to stop! Wouldn't that be great. For you to get so happy about an activity that you have been avoiding for so long would be phenomenal.
Wouldn't you like to see how this works. Pick a day and a time - oh, and of course, an activity. Put this "appointment" down on your calendar so you're being held accountable for it (even if it's just accountable to yourself). Then just do it. Even if you only do a tiny little bit. See how you feel afterwards. Journal about it. Then do the scheduling thing again and again until this becomes second nature to you!
Let me know how much motion you get!