My middle toes go numb after 30-min of moderate/vigorous elliptical workout. sometimes happens when in heels all day. overdoing it, or more serious?

Consider you : May have a neuroma. Seek treatment from a podiatrist.

Morton's neuroma: This is probably pinching of the nerve between the third and fourth metatarsals. Orthotics with a metatarsal pad can be of benefit. These can be made for high heels. Sometimes a cortisone injection reduces the inflamed nerve. I would recommend being evaluated by a podiatrist.