struggling!

I did 3.4 miles today. It was a stuggle. Couldn't seem to find the zone. It was so slow also.  I don't know why this is so hard for me. Years ago I use to do a minimum of 4 miles daily. When I was training to run a marathon, I was doing 50 miles per week so why is this so hard?
It must have something to do with being 40 pounds overweight, or being older, or having an arthritic back or all three.
I would like to run a bunch of 5k road races next spring.  Is this something I could really do or am I just having delusions of greadeur?
My dog will be 9 years old in october. She was born on halloween, the same exact day that my other dog died? Imagine that!
I can't remember if I mentioned what the vet said.  He looked over all the tests and he thought the seizure was just caused by a software glitch, whatever that is.  and they come in clusters like aftershocks from an earthquake.
He did say that she had a heart murmur which he thought will have to be treated eventually. He recommended an ultasound if we wanted to pursue it further and that is something we are considering atm but no one does them around here. We would have to drive 50 miles to an animal hospital that does them.  Probably going to cost us 500 or 6oo bucks! 
Also said she is a few pounds overweight and if she took it off would be a great help to her heart so we are cutting her back.
I will try to get some work done later. It's hot today and the humidity is horrible.
Hope all my friends have a great day!