Spreadsheet consulted again. I actually used my working heart rate as a reference point.
But as a %age of HRmax that would be about 80 - 85% which was close to marathon pace. I built up to this over a period of time, in increments of 5bpm. I got to the point that I didn't really need to look at my HRM.
It looked something like this...(hope this table works OK)
Mon 9 miles easy LA 130 Tue 9 miles LT 155 Wed 9 miles easy LA 130 Thu 9 miles LT 155 Fri 6 mile easy LA 130 Sat Sun 14 mile easy LA 130