I've been building up slowly and getting back to walking a few miles here and there. My main training will be later this summer, and prior to doing the camino I will be breaking in my boots by hitting the trans pennine trail. My plan is to just walk the section from Sheffield to above Hull in around 5 days as a warmup.
I think trekking up the odd mountain mioght be useful too. Im planning Snowdon and Tryfan for some point in July.
I might add I also hit the gym 3-7 times a week, often do 4-5 miles on the treadmill, but just have a problem with finding time to actually go out and walk 20 miles. [moan] Its not simple if you work 5 days and are generally wanting to spend day 6 recovering from day 1-5. [/moan]
I think when I start my camino, that will be some of the training. Their wont be the worry/excuse of work, just the road. I will simply walk as far as i can, then rest, then maybe walk some more or bail out for the day. After a few days i will fully get into it.