If your race involves a ton of vertical then you should probably consider the Mike Foote Big Vert plan instead of Luke’s plan. If you choose Luke’s Intro to Ultra plan then you can extend it’s duration by repeating some weeks or by doing the full plan for 20 weeks then restarting the plan at week 5.
Since you are just getting back into running after a lay off I suggest begin quite gradually and get your base mileage back to 25-30 over the course of 4-6 weeks. Once that feels comfortable then you start using the distance progression built into the plan.
Scott