@kiwiscott there’s some great advice above. I’d add that to get some good practice at riding rocks you may want to try a couple of the trails at Jungle. Specifically Boulderdash (essentially one continuous rock garden) and Warthog (a longer, flowier trail that, in the switchbacks, are unrelenting rocks). While there are certainly more technical trails in NJ than these, these two give you a solid dose of constant rocks without major climbing. Since your fitness is not the issue you can use these sections (do em both ways) to focus on the techniques described above.
