Yes, it's just being out of shape. It's not hydration or electrolytes. It's going to take time. Do more time at lower intensity and one-legged strength work. I like 1-legged knee bends from a chair, same number on both legs, the weak leg first, 3 sets say 5 minutes apart, say twice a week after a an easy or endurance ride. I ruptured an Achilles a long time ago. That calf never got as big as the other one, but I can do the same max number of calf raises of both calves. So that leg may never be as big again, but can probably regain its former strength.
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Results matter