Have done a number of 1,000 mile 3 day rentals with no issues across these 3 brands, return the car clean, and all will be fine; no issues at all.
Beware of Enterprise off airport locations and mileage charges. Always check the contract. ALWAYS get a print out of the contract.
Also find a different contract ID to use for National than that AmEx code- it increases the rates above the normal Emerald Club Members conract ID.