I'm honored.
I was a lower mileage runner due to injuries, but I responded to high mileage very well from a fitness standpoint. Higher mileage just meant I was on borrowed time before the next injury would roll around.
Making no other assumptions than what's been said so far, I wouldn't make adjustments to workout volume for someone running 55 miles a week versus someone running 65. The difference isn't enough to warrant much change unless there was also a discrepancy in fitness.
Keep in mind, if you're intentionally scaling back mileage you're also removing a stimulus that should be helping drive fitness. That doesn't necessarily mean you have to add something else to make up for it, but most athletes aren't okay with accepting that they are limiting their potential. There are usually easier places to reduce mileage. The first thing to remove should usually be doubles; they essentially cut your recovery time in half and aren't needed when aiming for lower mileage. Reducing /removing the least essential run. For us, this might be a run the day after a workout. 8 miles the day after a workout isn't there to make us substantially more fit, it's there to maintain and recover and with just a small amount with the idea that enough of those runs over time might contribute to general fitness. Last to cut clean are probably cool-downs or potentially warm-ups. The reality is that a cool-down could be done on a bike, elliptical, while swimming, or even walking just as reasonably to accomplish the same goal (provided you aren't using it for weekly volume like we do).
Just about any program can strip out 10 miles by tackling one or more of those things.
You'll also hear some people saying, "you can do any volume as long as you run it slow enough." Not to piss of an entire peoples, but I hear that alongside a lot of Tinman fans and I can't say I agree with it. There's absolutely some truth to it, running fast requires more force to be put into the ground. Hit the ground hard and the ground hits back hard, but repeated stress is still rough on the body even at slow speeds.
Getting back to workouts. This is the area I would try to retain as much as possible alongside maybe a long run. These days are the biggest bang for your buck and can easily be achieved without the accompanying higher mileage as long as you're smart about the progression; especially if you have been able to handle those workouts before.
Mileage can take a pretty serious hit after you've gotten in shape. When you've hit the point where you can tempo well and handle long runs feeling strong you can back off of mileage a ton before you notice fitness deteriorating provided you have the right workouts and frequency going on. I would want to see a steady diet of tempo running to keep some of the higher quality endurance always at hand.