So your not advising any workout under an hour long
Wow, was that the wrong conclusion to draw.
No what I was say was simply riding for an hour or so, no matter how hard, isn’t actually that useful in terms of specific XC training.
The two most critical sessions in my schedule are riding 2 * 20 at a set power level with 5 minutes rest in-between each interval, and 4 * 4 minutes, again at a set power level, with 2 minutes rest in-between each interval. Both sessions are way less then one hour.
Massive gains can be had from Tabata sessions which are 15 minutes or so.
Anyway at 15 I’d say join a club.