Your friend is right, full Marathons are not a logical extension of anything. Marathons beat the crap out of your body. Starting out with a Half Marathon is perfect, maybe even a couple of HM's.
You are a few years older than me (I'm 55 and started running 3 years ago), and like you I need the necessary rest days to recover. Nevertheless, most of the time I'll average 4-5 days per week. Long runs are always on days when I know that I can have the time to recover afterwards (for icing, stretching, getting my feet up). My goal has been to make sure that I'll still be able to run when I'm 75+. If something hurts, it's a sign that something is off and I need to address whatever it is.
Speed is something that will come with training and experience. In the meantime, your fine to just enjoy the run and build up the endurance. There are tricks to getting faster (other than stealing someone else's genes!), and you will figure those out.
Everybody has different reasons for running a Marathon, so just be sure that you are solid in yours. Mine was simply that it was something that was hard to do, but not impossible to achieve. If your goal includes running the distance completely, i.e. not walk-running, it is a tremendous thing. You will cross incredible chasms, both mental and physical, but if you train for it properly, you will succeed, have a great time doing it, and will want to do it again.
I had my struggles, and a struggle during my marathon, but I did it and can't wait to do it again. I had hoped to run 2 marathons in 2012, but had to deal with one injury after another. So now I'm looking forward to 2013.
I have a blog about my marathon training and running that I've been keeping for my Dad (so that he can participate with me vicariously) and you are welcome to read it:
http://runningthrutime.blogspot.com
Good luck!
-b
http://runningthrutime.blogspot.com
6/17/2012 AA HM 2:09:01 (2 weeks after DexAA, too hot, too soon)
6/3/2012 DexAA HM 2:00:31
3/11/2012 St Pat's 5K 0:25:42
11/10/2011 A2TT 10K 0:52:27
10/9/2011 Chicago Marathon 4:49:55 (cramped at 23m, but finished running!)
9/3/2011 Milford 30K DNF (cramped out at 14m)
6/25/2011 Solstice 10M 1:27:15
6/5/2011 DexAA HM 1:58:53
5/1/2011 BP10K 0:51:58
10/17/2010 Detroit HM 2:06:19
6/6/2010 Dex-AA HM 2:09:30