To achieve the 50% intensity target during a session, as already indicated I would put the client on a progressive tabbing programme whereby they started out with a target of consistently covering 2 miles in 30 mins (this will be fairly comfortable for the client given their predicted V02 Max, and a good starting point to get them out exercising consistently).
I would set the programme to increase the distance to 2.5 miles over a 4-6 week period based on feedback.
This would mean the client will be exercising at a speed of between 4 and 5 miles per hour, at an intensity of between 40 and 70% of their functional capacity.