8 mile run on day after triathlon

Probably shouldn't have done that. Today I was supposed to take a rest day, except for my yoga class in the morning. About 1:00 I decided I really wanted to go for a long run, 8 miles to be precise. The thermometer in my office said 89 deg which didn't sound too bad. So I strapped on my Camelback and my GPS and took off. I made the first three miles doing pretty well, but then started doing some walk breaks. It really felt hotter than 89 and I was thankful for the full Camelback of water. By the time I got to Planz Rd, about 4 miles in I could barely do more than walk. Essentially I was walking almost the last 4 miles of my 'run'. My lower abdomen is hurting, my inner thigh muscles are sore and my glutes are sore. I will definitely need to rest for a couple of days. It's hard to accept that my body just doesn't react like it did when i was 30. It takes me longer to recover from hard workouts. I have been reading Scott Jurek's book Eat and Run where he talks about how the vegan way of eating has given him more energy and enabled him to recover much faster than when he was eating animal protein. Sure Scott, that's just great for you, but you are not 66 yrs old!!