1) Hertz has the same type of old, high mileage cars, especially for customers on cheaper rate plans, even more so if it's the high season in your area of rental. 40k+ mi cars are relatively common.
2) Enterprise has always kept their cars longer. Once in a while, you could get lucky with a brand new car here and there. Now that they've bought National, it's less likely. National keeps the car for 30k mi, then gives it to Enterprise. The thinking goes, cheaper rates, cheaper cars.
My local Enterprise used to have a bunch of mid-mileage tiny Korean cars, most with NY plates, one or two borrowed from nearby NJ. They now have several beat-up Japanese and American cars with out of state plates. Enterprise neighborhood stores do not allow one-ways to be booked online, due to their small fleet size and tight utilization. These cars were one-ways retired from National.