Making time for myself

I was talking to one of my former classmates on http://www.odnoklassniki.ru/, telling her all about my life (we haven't spoken for almost 20 years). I was telling her how I spend most of my time with kids, and she said: "That's good, but do you have any time for yourself?" This question puzzled me. I thought about it, and answered that the time that I spend with kids IS the time for myself, because I enjoy it so much. And she said that I have to reconsider it, because "the kids grow up, you know." She knows about it better then I do - her son is 15. As for me, I don't think about it much yet, because this thought scares me.
Sure, I could easily take an exercise or yoga class, leaving the kids at the gym daycare, but I'd rather drive the kids to their activities and watch them learn and enjoy new things. In the evening I could read a grown-up book, but I'd rather snuggle with my son in his bed and read Greek myths or Shakespeare fairytales. I could find my own hobby, but I'd rather talk to my kids about different topics that interest them. Me and my husband very rarely spend evenings out, because even if we had a reliable babysitter, I like putting my kids into beds myself. And of course we could go on a "grown-up" vacation, leaving the kids with grandparents, but we never do that, ever. We tried, but our conversation always revolved around kids, and I didn't enjoy a vacation as much without them. It's much more interesting for me to go with kids, see how they react to new things, how they adopt to the new environments, what they like or dislike. I see the world through their eyes, and it's so much more interesting.
Well, so much for the time for myself. Truthfully, when the kids were babies and required my attention every minute, I used to crave some free time - to go shopping, read a book, or just go for a walk. But now, when kids are in school for much of the day, I don't crave that time anymore. I spend my "free" time cooking, cleaning and grocery shopping, waiting impatiently for the kids to come home so I can do things with them. Because time I spend with them is also the time for myself.
Is that healthy? Probably not - every "relationship" book will tell you that you need some time for yourself and some time alone with your spouse to keep yourself and your relationship strong. I wonder why I don't need that.
Before I had a family, I had many chances to have "time for myself" - many more chances that I actually needed. Growing up as an only child meant spending many boring hours by myself. Leaving my home for another country meant leaving my friends at home, and again, spending time wondering on the street, or in the mall, or simply staying home - alone. And I am not the kind of person who enjoys loneliness. I guess I've had enough time for myself before I had kids. But there is another reason. We are all growing older. Many people don't mind it, but I am struggling with this. Spending time with my kids means I can do "kid" things again and again - read a children's book, take a walk in the rain, play at a toy store. I'll have plenty of time to be by myself when my kids grow older and don't want to play with me anymore. And this is a very sad thought for me.