He can certainly move faster than my old boy!!!
My dog has periods of panting and sounding like he has something narrowing his throat, he has had these periods for a couple of years now and it comes and goes (he does it for a couple of weeks then it clears up). In fact last night I went past a friend and she said "sounds like that dog has emphasema". It does sound really bad sometimes and the first few times it really scared me as it sounds like a really bad asthma attack. I considered an x ray but decided against it as he is petrified of the vets and I think the visit would kill him.
I think as long as they are eating, asking to go out for walks and managing to sleep peacefully etc. then let them be. Jake is obviously still happy and alert. Remember he is the equivalent of about 95 in our years!! You would not expect to see a 90 year old jogging down the road would you?
Have you thought about a harness for Jake to help hold his back end up? This site has pictures (you can probably make one easily).
Walkabout Sling/Harness for Dogs & Cats-Hyperdrug
This article gives details of how to help them with physiotherapy:
Degenerative Myelopathy Member's Roster: Physiotherapy for DM Dogs. By James Colla
Sue