That's all pretty familiar, I started out very unfit, worked up to 10km and then got that under an hour, half marathons under 2 hours. I kicked it up about 6 years ago, more racing and smarter training, times came down a lot and managed a 1:31 half and a 3:31 marathon. I also started doing ultras and although they were much slower they did wonders for my endurance. 10 hours or more at peak training load sounds about right as well. A few weeks ago I managed a marathon PB of 3:27 off the back of a couple of months of excellent training, where everything went to plan, but it took absolutely everything, I had nothing left at the end. That's as well as I can train, I had the easy longs, the speed work and the other easy miles with plenty of cycling and a bit of strength work as well. My 5km/10km are gradually getting slower even with that quality training, I just don't think there is a way to get significantly faster for me.