I normally would say overtraining is the issue but I do think some firing of the muscles have changed. I do deal with some lower back arthritis/disc issues but I do not have any back pain. I do get pain sitting at times in my left butt numerous things I had checked with MRI and PT last year. It comes and goes but does not hurt to run or ride as such, and mostly only after I am done running. At 53 I just have cut back and realize I cannot keep it up. If I simply ran I am sure my training pace would improve but I love the bike. I do not train for anything anymore did marathons years ago but not need to do that. I just run and ride for stress relief but frankly I would like to every once in awhile run a few hard sub 8 minutes miles. Yesterday I ran 7 miles in 64:45 about 9:15 pace. I could have with not much more effort run another 4 miles at that pace or close to 1/2 marathon, but ask me to run 3 8 minute miles I a row I just could not, nothing to do with have the wind just slow legs.