Here's the most common and simplistic formula to measure effort based on heart beats:
220 - your age = maximum heart rate (MHR)
For Kurt, 220-48=172 MHR.
77-78% MHR for marathon = 132 beats per minute
See Coach Bensen's site (below) on effort-based training. He says 77-78% MHR is realistic for running a marathon, so we need to get our fitness level in that range for 8:45 pace. Once heart rate and pace matchup, and we put in the longs, we're realistic for the sub-4.
INTERVALS: He also suggests never to run your 800m repeats (intervals) over 90% MHR (155). So anything that approaches 155 MHR would give you a maximum VO2 benefit, without risk of overtraining. Recovery jogs of 400 meters should be around 70% (120 beats).