This plan is designed to prepare runners for a trail running race of 25 to 45 km-effort. "XS" is referring to the International Trail running association (ITRA) classification where 1 km is added to the total distance for every 100 meters of elevation gain.
The plan includes strength training in the form of one weight lifting session and one hill sprints session per week.
This plan is a variation of the S, M and L plans. The intervals and strength sessions are mostly the same for each of these plans. Only the volume changes.
The session is attached.
This is an easy run including 5 minutes at threshold pace to determine your threshold heart rate.
10' warm up
5X30'' uphill sprints r = 1:30 walking recovery
10' cool down
Running on a course that matches your race in terms of elevation change and technicality is ideal.