I've read On Writing three times and I don't remember his advice for new writers being quite that tough. As I recall 4-6 hours a day is the time he himself dedicates to reading/writing every day, but obviously that's as a successful full time writer. I think the recommendation for aspiring writers was two hours writing a day, six days a week (fairly sure he allowed for one day off a week). If you've got a full time job, family, kids, four to six hours daily is just not going to be possible. I've been trying to follow the advice as best I can for just over a year now during which time I've been working on my first attempt at a novel, and you know what? Two hours a day six days a week has proved to be beyond me as well. A really good week I maybe manage five to six hours writing in total and the same again for reading, both done on trains as I commute to and from work (about the only free time I can find).
I think the piece of advice you have to take to heart is "Don't come to it lightly". The important thing is that writing becomes a habit and it becomes something that you have to do, so that if you go too long without firing up the laptop you start to feel uneasy, edgy. You need to make it an addiction that you need to feed. That's what I took away from On Writing anyway.
I do a bit if running and it's a similar thing for me. I started running over ten years ago for reasons which become less and less clear to me each day, but I know when I did start it was really bloody painful. But if you can stick with it, it slowly gets easier and you begin to develop into a better runner, and then you get better and better and better until one day your hurtling along and you suddenly realise that you're enjoying yourself and you feel like you don't ever want to stop. I still hate running sometimes, still have days when my legs are screaming at me and it's cold and wet and I'm exhausted, but it doesn't matter, I pick myself up and go anyway. There are two reasons for this. One, I know that however bad I feel before a run I will feel worse later on if I duck out of it. Two, sometimes it all comes together perfectly and it's the best thing in the world when it does.
I think that writing is something the same. When it all comes together it can be joyous, but you don't get to have those days without putting the miles in. And to put the miles in you have t be disciplined and push yourself to do it regularly and often.