What??!?!?
....build up to 6 days a week over several years??! Dooooom sayer!
If you can run 6 miles this week you could easily run 9 the next, then 15, then 20 etc, and if you're only running 6 miles a week at the moment but want to run 6 days a week, how hard would it be to run 1 mile a day? Really??!
The following week you could run 1.5 miles a day, thats nothing, you wouldn't notice it, yet in week you'd have gone from 6 to 9 miles, do this sort of increase for a few weeks and you'll be doing 35 miles a week easilly, most people on here do more than that a week wth thinking about it, some do that in one training run.
So Nicola, you'll be fine, ignore the doom and gloom merchants and 10% is an extremely conservative increase (one size fits all approach) a 60 yrear old couldn't increase as fast as a 30 year old, slower recovery.
The problems come when you run 6 miles regular, then try and run 15 in one step, which you might make, then think great I can run 15, now i'll try 20.