I also started horseback riding again, and was so surprised by the fact that my inner thighs were so weak! I took a 5 year break from horseback riding, but I thought that running and cycling would have kept me strong, but I guess not!
But now that i'm getting those muscles back, I also find that i'm a strong cyclist. Funny how it all works out in the end.
Now if I could only have a stronger core...