Hi Veronica!
For race viewing, each race is different. None of the races require family to purchase packages simply to view the race. But some races have more spectator-friendly areas than others. The Tower of Terror 10-Miler would be the hardest to view other than at the finish line which would require admission to the after-party.
The Disney World half and full marathons are very spectator friendly, as is the Princess half marathon. All fans are permitted into the Magic Kingdom to watch the runners run down Main Street U.S.A. No tickets are required. The packages do offer reserved seating, but you don't have to have them. You do not have to buy packages at the same time you register. You can always add those later.
My suggestion would be to find the race you are looking for on the
runDisney website. There is a tab on each race page with "spectator resources" that will outline the details.
If you have a particular race in mind, please let me know and I can answer it for that specific weekend.