Athletes over 50

Is it possible something else has changed?  Ankle flexibility that has your feet slapping the pavement or landing on your heel rather than the ball of your foot? 

When was the last time you changed shoes?  Old shoes, or new shoes can make for newfound soreness.  Even if you bought the same shoes you previously used successfully, the company may have changed things on you. 

I was a member of the Gray Guys/Girls Maturing Triathlete Mentor Group for a few years.  There was a lot shared on how the game changes in your 50's, how it changes in your 60's, etc.  It might be worth dig up some of these old mentor groups and skim through them to see what was shared on aging with endurance sports and the advise that was given. 

I don't remember a lot of the tips on "aging with grace" but if you are consistently sore after running you need to make some adjustments.  There are things that you used to do that didn't make you sore when you were younger that will make you sore now.  Recover times are longer, strength training is more important, form is more important, going at too high an intensity will not be as forgiving, etc.  Note: I got my first gray hair at age 37.  I was one of the young guys in the gray guys/girls group.  I haven't hit 50 yet.

One likely possibility is you need to increase protein consumption.  Even though nutritional guidelines are the same for all adults, we actually need more protein as we age.  I've had athletes over 50 who started struggling with soreness and recovery after training, and it resolved very quickly by increasing protein.  How protein is consumed can make a difference too.  Always try to consume 20+ grams at a time (but no more than 30g).  20 grams gets you over the leucine threshold to promote increased muscle protein synthesis.  Every 3-4 hours throughout the day works best.

Another less likely possibility is your running intensity.  Take a hard look at intensity.  Has the intensity of your easy runs crept up over time?

54 years old here and have had the same experience - aging sucks. 

There is some great information in Joe Friel's book Fast After 50 . 

It's likely just advancing age.  DOMS - Delayed onset muscle soreness is a thing for everyone.  Generally speaking, you'll be sore after an especially strenuous workout a couple of hours to several hours after the workout concludes.  That soreness will last a day or two (normally).  Unfortunately, as we get older, the soreness often begins sooner with less strenuous activity and may last a bit longer.  Tending to proper recovery following a workout will help.  That means taking in carbs and protein in a 4/1 ratio following a workout, gentle activity (like walking, an easy swim, etc.) to flush any lactate, and rest.  Chocolate milk generally has that perfect 4/1 carb/protein ratio and is an excellent recovery drink.

Hope that helps.