I've experimented with various and find 9:1 works for me, and yes I'd do it from the start. If it gets tough towards the end you can reduce the running intervals, but personally I don't like walking for more than a minute as I find it hard to get going again - even if that means I'm down to 1:1.
Larri - the answer is - it depends. 4 hour marathon - no problem. My PB's 3:49 and I ran my Comrades qualifier (Barcelona) in 3:56. There are plenty of people with much slower marathon times than 4 hours - it's not about speed, it's about endurance (and a high pain threshold).
Distancewise, I had only run further than marathon distance twice before I started my Comrades plan, one 30 mile race and one 36. So I wouldn't worry about that either. My longest training run was 37 miles and I've seen plans that don't go over 30.
But that said, I started my training off a fairly solid endurance base - I had been in training more or less solidly for 4 years, running at least one spring and autumn marathon each year. So I had quite a lot of miles in my legs before I started, and I think that helped. I wouldn't have liked to attempt it off the back of one marathon.
If you are thinking about signing up, don't leave it too long though - entries close at the end of November
Emmy - I don't specifically do an extra mid-training cycle race, but if the opportunity presents itself, I'll have a go.
KS - could it be a non-running-related physical issue? Have you had iron levels and stuff checked? And, before Dannir appears and suggests it, is there a possibility, shall we say, of the patter of tiny running shoes?