How much you're able to ride is pretty subjective on daily life and such, kids, work, marriage, etc. The mantra of riding as much as possible to get better is a great starting point. Along with that, if you wish to train for performance, set a goal for each ride, no matter how detailed, sounds like you do some of that when you choose which ring to stay in.
XC racing is going to have a lot of interval-like attributes so doing some interval training will help, which is usually easier to do on the road.
Big thing I'm finding out is that fitness isn't everything. I have a very solid engine from training for road races but I pretty much suck at high speed technical corners and any technical downhill like sections. If it's downhill and there's a turn, I consider it technical because I simply am not that good at it yet. That's an area of focus fro me because that's my weak spot. I suggest finding what you're weak at and working at it. The limiting factor should help get you a lot of time in the races over improving just what you're good at. Don't ignore your strengths, but you're looking more for maintenance in those areas since there should be better time benefits from improving the weak areas, with some exception, like if a trail has no technical section, training to be a super technical rider won't help a whole lot.
Weekends are a great time to get in the longer miles to build up the base fitness. I usually aim for 3-5 hours of riding for days on the weekends (I have the time, you may not. I will ride twice a day often on these days). If you're near to a group getting in with a group ride is usually beneficial because you can see what the good guys are doing, you can ask questions, and get tips on how to get faster.
For reference, I usually ride 10-15 hours a week over 6 days of riding (usually) with a mixture of road and mountain depending on what races are coming up. I focus on road and cyclocross and then mountain bike for fun. I once held a USAC coaching license but let it expire because it kept feeling like they keep wanting more money for the education (in terms of CEU's) I can get for free from training peaks webinars, reading books, and other sources. Anyway, shoot me a pm if you want a sample training plan or something as a reference to go off. There are others on the web as well that you could search for as well.