I think about this question a lot; the planning for a long trip can seem overwhelming and sometimes so far away, but there are some small things you can do in the lead up to make your travel happen.
Set a goal
What do you actually want to do? For me it was "travel and explore, be active, not waste life while I'm young enough to enjoy it".
Body
Work on your fitness in preparation for your trip, this wont happen overnight so get onto it early, start walking up inclines and stairs...there's a lot of hills out there.
Mind
Focus on achieving your goal, remember that feeling of freedom when you have any setbacks, or have a bad day, remember why you wanted to do this.
Health
This is sometimes overlooked in the planning phase. If you have any health issues you have been avoiding, get them sorted out so you are well enough to travel on long trips.
Money
Assess your finances, what are all your expenses now? What will they be when you travel? Have a plan on how you can fund your trip. Can you make some extra money in the lead up to leaving? Can you line up work for 2 months into your trip?
Social
Will you be sharing your trip while you travel? You can start setting up your social accounts now in preparation. Think of a name, and get a Instagram, Facebook and YouTube account.
Research
Planning where to go is half the fun! You can start collecting camps and activities that appeal to you, can be as simple as a screenshot on your phone, or noted down in a book, even dots on a map like Wiki Camps or Google my maps.
Get Inspired
Following other people who are already out on the road can be a great way to get ideas, find cool locations and activities and tours you want to do, Instagram is easy and good for finding travellers.
Vehicle
Will your current vehicle be suitable for your travels or will you need to upgrade? You don't necessarily need a brand new vehicle to do the job, check your van weights and go from there.
Caravan/Camper
What are your plans for towing? Go have a walk through some van yards and take your time choosing something right for your situation and budget, it doesn't have to be the latest and greatest to have a great time.