Can't advise on the length issue. Others are correct about walking, as the traffic on the loop road is horrendous...unless..you stay in the campground on a Tues and Wed night. Wednesdays for the summer they shut down the road, and it becomes walk/bike only. THAT is the best way. The ride isn't all that long, and the hills are pretty manageable even for occasional bike riders. Lots of stops to take breaks, see the historic sites. And the views are so much better at a slower pace.
2012 F150 Eco, 4x4, SCrew, Max Tow, HD Payload
2017 Grand Design Imagine 2670MK