This was a much easier task then booking the return ticket. I only did it afterwards because I was afraid of committing to the trip. The fee to reinstate miles on United was $200 last time I checked, so I wanted to make sure I wasn't going to change my mind. Now that I had my return flight, I had no choice but to go. Having a return date of April 28th, I tried to give myself at least two weeks to complete the Camino from Porto to Santiago. The only date available, with a somewhat convenient route, was April 5th - Easter Sunday.
Trip to Lisbon on United Airlines (San Francisco - Frankfurt - Lisbon) = 30000 miles + $111 in taxes/fees. Good to go!!
Edit: I forgot to mention that this flight was operated by Lufthansa Airlines.