Rebuilding cardio fitness

A wrist injury about 15 months ago was a major setback in my cardio fitness— I’m an avid mountain biker. Back on the bike over the past 6 months I am so slow, I can’t find a way to get back to where I was. If anything, I grow slower and slower. I lift weights 2-4x per week and try to ride 2x but it varies, and it’s hard lately mentally bc I’m so discouraged at how slow I’ve become. I’m 46/f.

Cyclists/runners/other cardio-driven athletes, any tips on how to build cardio fitness back in this chapter of life?