The biggest mistake kids make is rushing back too fast and injuring another part of their bodies. It takes a good 6 weeks to be healed enough to begin running at all. You didn't mention what kind of x-training you're doing or if you were booted (Cam boot), etc. but I'll assume you're out of a boot and doing non-weightbearing x-training (biking, swimming, pool running). I'll also assume you're in PT of some sort. If not, find one with an alter g, and one that works with runners. Assuming that, my suggestion is to continue that for the next few weeks. When your doc or PT gives you the ok, if you have access to an alter-g, then that's a great start. They should be able to start you on that soon if you're not having pain at the sfx site. They can gradually increase the percentage of weight until you're 100% weight eating. It's a great machine.
Most PTs will have you start back running very conservatively. Like alternating run/walk for one minute each, for maybe 5-10 minutes, then x-train the next day, then try running the same again for a week, every other day. If still no pain, they'll slowly increase the running to walk ratio, then increase the frequency. It'll be a bit before you can do the 30-40 min runs, especially at your probably decent previous pace, just because you've been off land for a bit. Pool running is a great way to make up for the miles you'll miss. Run done and then hop in the deep end of the pool every single day. It's a great way to do doubles even after you're all healed up. Just be sure to get your heart rate up like you're land-running. Really use your legs and arms and mimic running as best you can, using a flotation belt. Some areas have classes so you're not bored out of your skull. Sprint in the water. If you go slow, you'll lose cardio. Biking is another good alternative, but you have to bike long and pretty steady/fast to get any benefit. A combo is a good idea.
In addition, now is a great time to reassess your core and all biomechamical stuff. In most cases, sfx are caused by ramping up mileage too fast or doing too much speed work. I don't know if you're male or female, but make sure you aren't undernourished either.
Oh and expect ghost/phantom pain as you get back to running. Ice and compress after runs and don't take nsaids. If your pain goes away after a little bit, that's ok. If it lasts, you might have overdone it. But it doesn't mean you refracted it. The callus that forms to heal isn't as smooth as regular bone and it just takes time to settle down. Listen to your doc, your PT, and your body. It's best to err on the side of caution so you're healthy at the end of XC and not back on the injured list.
TL; DR: take it slow, pool run like it's your job, strengthen your core, listen to doc and PT, find alter g, and start running very slowly and carefully. The important stuff is in October and November anyway.
Good luck!