Sorry you are having problems
First are some very general internet thoughts that do not take the place of legitimate medical advice.
'Coning of vision" , or loss of peripheral vision, occurs in acute and severe hypoxia situations, like pilots. It is not a classic manifestation of simple altitude illness (AMS)
At only 8,000 ft height, there is not enough hypoxemia to do that, unless you have something else going on: (A) migraine equivalent or some other vascular phenomenon. (B) Cardiac arrythmia (despite your successful hiking /running career) because as you say the symptoms improve with stoppage.
Don't let your PCP blow it off as old age or pressing too hard. You need a specialist. I think a cardiologist and a neuro-opthalmologist both may be needed here. Better be safe about this.
