I've always been a proponent of mileage but what is the purpose of running 60 miles a week over 40 miles a week if you have to run slower to do it? Isn't the idea to run faster? People get obsessed with numbers and that's when you start have problems. A 20 mile jump would be a mistake at this point I think, it sounds like you've got the right idea with bumping up a little bit but not too much and if the pace suffers or you notice anything that doesn't feel right don't hesitate to go back to 40. You usually want to wait until the current amount of miles or intensity no longer provides you with the stimulus that you need before you increase one or the other. You've got plenty of time to get stronger and to improve, you're only one year in and you're seeing improvement so it sounds like things are going well. I'd listen more to the "don't fix it" guy than the, just run more miles guy. Don't get obsessed with numbers, be more concerned with improvement, something that sounds like is coming pretty quickly at this point.