Okay Francois, I just spent two years in Arizona, so I know all about rocks and cacti, and crashing into said rocks and cacti, so I think I can speak with some experience. I also know all about working on skills for the over-20s! It was when trying to learn to wheelie at the age of 30 that I started rueing not having mispent my youth trying to do these things…
Some of the advice given by others here can work well; it can also be disastrous. For instance, trying to encourage a technically-inept rider to keep up with faster, skilled riders over technical terrain is a recipe for crashes since the faster riders are doing things subconciously that the inept rider has yet to understand. Additionally, what’s the point in riding more unless you can learn from your mistakes? So, my advice:
a) You’re in the desert, so wear full-fingered gloves.
b) You’re in the desert, so run Stans in your tubeless tyres. With all the punctures from cactus thorns and pinch-flats from rocks, this is a must for an enjoyable experience.
c) Perhaps consider wearing some knee/shin protection and maybe elbow protection; cuts, bruises and grazes are an inconvenience that can be avoided. Also, the protection can help with confidence. Confidence makes you more relaxed, and being relaxed is critical for navigating technical terrain.
d) Purchase “Mastering Mountain Bike Skills” by Brian Lopes and Lee McCormack. Read, digest, practise. This is by far the best text out there.
e) Rather than trying to keep up with experts, try to find an expert who can help you; not so easy to find, but would you encourage a novice swimmer just to try to keep up with elites, or send them to a coach to explain how to swim better?
f) Treat it experimentally; if things go wrong, try to work out why. Take things slowly and build your skills in your own time. However, also test your limits regularly. See if you can watch people riding things that you think are unrideable.
g) On technical descents, the most important thing is to keep the front wheel rolling. That means laying off the front brake in rocks, and having a certain minimum speed. So brake on the smooth parts, let go on the rough parts. Learn to lift the front wheel up/over rocks. If the front wheel keeps rolling, the bike, and you, should make it through. I cannot emphasise this enough!
h) General body position should be fairly upright with elbows bent out to the sides, allowing the bike to move around under you. Do not try to ride in your road bike position (sounds obvious, but a mistake I made for many years). The Mojo geometry should help with this.
i) Weight placement: forward on climbs, back on descents. On flat, twisty singletrack press forwards into the bars on the bends to help keep front wheel traction. You and the bike should be moving around all the time, dynamically.
j) Relax. Easier said than done; should come with time. Learn to let the bike move around under you.
k) You’ve got a great bike, but if the suspension is set up wrong it will be lousy. Find expert help/read up on set up, and experiment with shock pressures etc. Keep a log of set-up to help you hone in on the optimum.
l) Belief is everything.
There is of course more, but hopefully that will do for now…