I always book a single one-way flight to start my trip. Bangkok, Cape Town, Cartagena, etc. Just get to one end of a continent and plan on spending as much time as you need moving across it.
Dates on the calendar are the biggest source of unhappiness when you're on the road. There's nothing worse than having to leave a place you're really enjoying because you have a flight to catch two countries away next week. You'll never expect to spend six months in the place you end up spending six months in. If you book ahead of time you're guaranteed to have to either blow off your expensive ticket or miss out on what would have been the best part of your trip.