Like Dot says, it's just muscles that aren't always used in everyday life. My advise would be to do it as much as you can so your body gets used to it and you don't ache so much afterwards!
It takes six months to get fit.....and only six weeks to lose it. So...if you haven't had sex for six weeks it's like starting all over again....Then you think...'ouch...how the hell did I get my leg in that position before'.... :-)