If you are 40 years old, running 30 miles per week, and want to get to 90, I would be happy to put money down that you'll get injured in the process no matter how slowly you try to do it. I say work your way slowly up to 60 and then stop there. If you feel a need to do more, then it is time to add some cross training activities.
Here is the reality -- VERY few runners ever really run 90 miles a week for any length of time. Most who try end up injured.
I would honestly wonder how many runners there are in the entire US who are 40+ and run 90 miles a week for say, a month or more. I would be very surprised if there are more than 100.