This is a popular route for mountain bikers and off-roading vehicles to go up to Palomar Mountain, but you can also run it! I parked where the AllTrails map took me, just where the road turns into gravel/dirt. I parked on the side of the road there and started a little after 9am. This route is about 8.5 miles uphill and then you turn around and go 8.5 miles downhill. I did not see anyone on my way up, but had some motorbikers and cars pass me on the way down. It was a long and tough climb up, I am glad I did not start super late, it would've gotten pretty hot. I refilled water at a spigot by the bathrooms at the top. They have a lovely picnic table at the top, where I stopped to have a snack before descending back down. The descent down flew by so fast compared to the long trek up and was happy to reach my car again! If I were to do this again, I would bring my poles for the uphills. The dirt road is pretty exposed for the first 7 miles up, so bring sunscreen and a hat! Some amazing views on the way up and it was so cool to see how the plants and biodiversity changed as you gained in elevation. A good route if you are training for a hilly ultramarathon, or just want a good hike! Official turnaround for the FKT route is the park entrance gate.
Here is the AllTrails link in case anyone wants to take a look at it:
https://www.alltrails.com/trail/us/california/nate-harrison-grade--4
