Pack and hydration

I'll have a trail-running race next week around Ikoma-Shigi mountains. I was thinking to go with a 500mL water bottle with me, but was wondering if it is a good idea in case of unexpected weather. Yesterday, I bought a light-weight pack and hydration water reservoir for another option. The pack is Gregory's Stimulus and the reservoir is Hydrapak's 1.5L. Both are great and I'd like to try them during this weekend before the race.