Thanks again to everyone who weighed in with advice and sharing their own injury experience. (Went for a 4 mile run a week last Friday and woke up the night after with pain all over right leg, centred on the knee). Just thinking out loud again ...
Had an interview with a Nurse Practitioner this week - didn't examine my leg. They referred me to their physio service who should contact me to triage my referral. I'm debating whether to look around for a private sports physio as that may be much quicker. The NP said they would exhaust other options before considering arthritis / x ray the knee.
In the meantime I'm now up to walking 2.5 miles at a time without pain, and continuous pain has disappeared from my knee. That has allowed to me feel a lesser pain more clearly which appears to be in the middle / lower right inside thigh, in the area of the quadriceps, I think. It's more tender than painful, and feels like I should take it easy on that part of the leg.
When I was younger - playing squash and football - I would have dismissed this as a muscle strain, and just give it a few weeks to heal. 3 miles into that 4 mile run I did feel a twinge in this area of the leg.
Off for a slightly longer walk this morning and see how the leg does!