The training for swimming and running I do would cover me to do a regular triathlon, but the cycling does not.

Having said that, if I were a road triathlete I would spend more time doing specific training for road as well. This would include more swimming, more running and probably less cycling.

If you race Xterra, the biggest factor to be aware of is the huge difference between riding in aero position and riding a mountain bike. You are not adequately prepared for a time trial position riding your mountain bike. You would need to spend some time on it getting ready for your road triathlon to really feel comfortable on the bike.

Other than that, you can use one or the other for really good training, but be aware that in order to be really good at either, you need to focus.