If you want to do focused training, the trainer is your best investment. You can do a lot over the winter with 3-4 one hour trainer sessions, and one long outdoor ride.
You've got to be disciplined though, and it's not a lot of fun for most people.
__________________
You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.