Hi everyone, I raced a CX race yesterday. Today, my legs feel great, too great?

During the race, I was pedaling with high cadence, I was forcing my exhale and breathing well, and my legs were burning in the zone. Overall, it was my best race of the season so far, but I wonder; could I have pushed it harder? Since my legs feel so good today, maybe I wasn't giving it my 100%? The main issue during the race for me, was my back/lower back. It was becoming so sore and fatigued that sprints and hills out of the saddle were becoming slow and sluggish. Even cruising and pedaling to rest I could feel the burn in my back.

What do you guys do to combat this weakness? What will help me most?