@Kayakgirl I think it is more stretching rather than strength building that you are asking about. First things first, always stretch before and after playing, especially if you’ll née the stretch like todays lesson. And don’t feel bad, it’s difficult for many of us. Your stretch will get better just from playing, but it takes a while. The type of stretching agony is doing is more drastic. Three things are important, stretch before and after, 2) don’t force a stretch.. you could pull muscles or tendons, and those can be debilitating. I had to hang my guitar up nearly 6 months from tendonitis and tennis elbow, which happened 2 Christmases ago when doing a song with very wide stretches. 3) There are exercises you can do.
One exercise I’ve used is to place pinky of fretting hand on the 8th fret. Leave that finger on the 8th fret and the place your index on the 7th, lift the index, place it on the 6th, lift the index and place it on the 5th. Do that until you can do it effortlessly while your pinky remains in the 8th fret and the index lands well within the 5th. Then move down a fret with your pinky to the 7th and do the same thing with the index to the 4th fret. And keep going. Do gentle stretches, not forced. You’ll be able to stretch further and further.