March is very early if you can't ride outside during the winter. I'd say a trainer would be much more effective than spin class for long-distance cycling. You'll want to increase your aerobic endurance base to match the length of the days you'll be riding (ie 6 hours, 8 hours, etc). You can do this with a variety of fitness mediums (not just cycling).
BTW, many who ride cross-country consider the first week of touring to be their most important training week. This would seem appropriate with you since you won't be able to log many outdoor hours prior to the trip.
Make sure your bike fit is completely dialed in before your trip.