As of September 2022, Mammoth Cave includes a total of 426 miles (685 km). In the IUCN evaluation (1981) the total cave system was listed at 190 miles (306 km). Today, 50 years after being designated the longest cave system in the world, Mammoth continues to expand with new discoveries (annually additional mileage of cave explored is published in September). This means the known cave system at Mammoth has more than doubled since inscription.

My first visit was in the Spring of 2009. We camped, we crossed the Green River via ferry, and we took the Historic Cave Tour. This Fall (October 2022), I re-visited the park for a couple reasons. Firstly, I found myself irresistibly close by for a conference in Nashville (1.5 hrs drive). Secondly, my pictures from the first time were not great (some were even lost). Lastly, with so much tours (reserve on recreation.gov), I felt the caves deserved at least one more tour (Domes and Dripstones).

Now Kentucky is considered #1 in America in physical inactivity. Perhaps this is why the park rangers have the stair count memorized for specific caves, for optional sections, and they repeat this information often. A two-hour tour is not especially strenuous, unless you have lung issues, knee issues, or rarely take modest walks or hikes. Anyone between 10 and 50 generally should not have issues and those ages are by no means benchmarks, a reasonably fit retiree should have no issues, but they should be cautious on the wet surfaces and stairs. As Els has pointed out, there is a bit of "hokeyness" to these tours, which fits with the audience they are catering to. 

Do NOT skip the above surface, because as IUCN notes its very much part of the OUV.

"Outside the cave, the karst topography is superb, with fascinating landscapes, luxuriant vegetation, and abundant wildlife, all of the features of a karst drainage system -- vast recharge area, complex network of underground conduits, sinkholes, cracks, fissures, and surface and underground springs -- are found within the site." - IUCN July 1981

My personal favorite hike in the park is Cedar Sinks Trail (1.8 miles/2.9 km). Turnhole Bend Natural Trail (mentioned by Els) is located nearby and is worth the short excursion. There are also plenty of trails of interesting formations just behind the Visitor Center. The larger backcountry trails are on the other side of the Green River. Since the caves are the main attraction, they are busy, many of the hiking trails however are not (this is to your advantage). There are also cultural sites throughout the park, including up to 80 Christian cemeteries or burial sites (some maintained).

There are reasonable and comfortable accomodations in Bowling Green, Park City, and Cave City. The park also offers reservable campgrounds, which I utilized after a one night stay in Bowling Green in 2009. Back to 2022, I rented a car from Nashville at 4pm, drove the 1.5 (2 hours with traffic) that evening and spent the night in Cave City. I spent about 6 hours in the park the following day before returning to Nashville airport (with my car rental). Note that the park is free, but cave tours cost a fee.