What Is Suggestion For Thigh Muscle Pain After A Long Run?

I am a 55 yo male, and have been running for nearly two years. Have run half marathons, 10k, 5k, etc. I started running and changed my diet extensively to lose weight after back surgery. I lost nearly 40 pounds and now weigh the same as I did in high school. My question is this, for the last several months, instead of feeling the high I normally would following a run of 5-7 miles (usually 5 days a week), within 30 minutes of completing my run, the muscles in my thighs and somewhat I my calves becoming extremely sore, as if I hadn t run in years. It is painful to walk afterwards, but not so much that I cannot function. The soreness stays with me to a degree most of the day. I consume carbs and protein within 20-30 minutes post workout. ANY help you could provided would be greatly appreciated. I desperately want to enjoy running again. Thank you so much. Steve